National Academies Press: OpenBook

Guide for the Geometric Design of Driveways (2010)

Chapter: Chapter 5 - Geometric Design Elements

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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
×
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 5 - Geometric Design Elements." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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24 This chapter sets forth geometric design concepts and guidelines for various driveway design features and components. Exhibit 5-1 suggests that driveways created as afterthoughts are less likely to perform well. The design of a driveway should be integrated into and take place during the design of the overall site. Before the overall site design is finalized, it may need to be adjusted and readjusted so as to have an acceptable driveway design. Exhibits 5-2 and 5-3 list geometric design elements that a designer may need to consider; not all elements will be present in every situation. This chapter groups some of these driveway geometric elements into the sections listed below and presents specific guidelines and suggested dimensions: • Driveway throat transition geometry • Driveway width and number of lanes • Median in driveway • Right turn channelization in the driveway • Channelization in the street • Cross slope • Horizontal alignment • Intersection angle • Space for nonmotorized users • Driveway edge and border treatments • Clearance from fixed objects • Length • Driveway grade (sidewalk cross slope), change of grade, and vertical alignment • Sidewalk cross slope (driveway grade) • Roadway-driveway threshold treatment • Drainage of surfaces occupied by user groups • Auxiliary right-turn lanes Presenting separate design guidelines for every conceivable combination of factors would make a publication unwieldy and overwhelm the user. For instance, when discussing the minimum con- nection transition radius needed for a residential driveway, not only is the width of the driveway important, but the needed radius is also affected by the width of the roadway and the absence or presence of on-street parking on one or both sides of that roadway. Therefore, the authors have presented recommendations suitable for more commonly encountered scenarios. Some of the guidelines may not apply to unusual situations. C H A P T E R 5 Geometric Design Elements

Geometric Design Elements 25 Exhibit 5-1. Unacceptable driveway designs. Shared Elements, Surroundings 1 Illumination 2 Conspicuity (to visually detect an element at a distance) 3 Sight obstructions Driveway 4 Width (maximum and minimum; sufficient for ped. refuge) 5 Lanes (number, width) 6 Median in driveway: (absence or presence) 7 width 8 type (raised, flush, depressed) 9 nose-end recessed from edge of through-road 10 Cross slope, cross slope transition runoff 11 Horizontal alignment, curvature 12 Connection depth (throat length) 13 Traffic controls or other potential impediments to inbound traffic (inc'l entry gate) 14 Paving length (applicable where have unpaved driveway) 15 Onsite turnaround capability (where backing into roadway is undesirable) 16 Driveway edge (edge drop off, barrier) 17 Space for nonmotorized users (e.g., pedestrian movement parallel to driveway) 18 Driveway border treatments (sideclearance, sideslope) Vertical profile 19 grade (maximum and minimum) 20 change of grade (grade breaks) 21 vertical curve design criteria 22 Vertical clearance (from overhead structures, utility lines) 23 Drainage (separate from intersection drainage) 24 Other special situations (e.g., railroad crossing, trail, bridle path, etc.) Sidewalk-Driveway Intersection 25 Sidewalk cross slope (i.e., driveway grade) 26 Path definition (e.g., visual, tactile cues) 27 Crossing length (i.e., driveway width) 28 Angle of intersection with driveway: flat-angle (turn angle < 90O); right-angle (turn angle 90O); sharp-angle (turn angle > 90O) 29 Bearing of sidewalk relative to street (i.e., sidewalk diverging from, parallel to, or converging with the street) 30 Grade of sidewalk (i.e., driveway cross slope) 31 Vertical profile of pedestrian route (abrupt elevation change: max. 1/4" ) 32 Sidewalk-driveway interface treatment: i.e., detectable warnings for visually impaired (e.g., truncated dome) (only at certain locations, inc'l. at signalized crossing; refer to guidelines) Exhibit 5-2. Driveway geometric design considerations that may be within the control of the designer. (continued on next page)

26 Guide for the Geometric Design of Driveways Sight Distance and Conspicuity Two considerations that are frequently part of the discussion of many design elements are sight distance and conspicuity. There are many types of sight distance. The basics of stopping sight distance and intersection sight distance are explained in the AASHTO Green Book (5-1, pp. 109–114 and 650–676), and an understanding of these basics is a mandatory prerequisite for anyone designing a roadway or driveway connection to a roadway. The designer should check that walls, wide utility poles, veg- etation, or other objects do not block the lines of sight that a bicyclist, driver, or pedestrian needs to maneuver safely. Conspicuity is the attribute of standing out so as to be noticed or observed. As applied to drive- ways, conspicuity means that users (whether bicyclists, motorists, or pedestrians) approaching the driveway can detect and recognize the presence of the driveway far enough in advance so as to make any needed adjustments in their travel trajectory or speed. Also, as the user either on the roadway or on the “private side” nears the driveway, the user can detect the precise edge or Exhibit 5-2. (Continued). Tr affic Controls (for driv ew ay ve hicles) 48 Driv ew ay -road wa y intersection co ntro l ( none, yi eld, st op, signal) 49 Tu rn restri ct ions 50 O ne-wa y operation (one- wa y, do not enter ) 51 Mark ings (pavem ent, delineator s) 52 Ot he r Road wa y in Vicinity of the Dri veway 53 Right-turn lane attributes: (absence or presence ) 54 lane width 55 lane deceleration, storage length 56 lane entry transition shape 57 lane offset 58 Left-turn lane attributes : ( abs enc e or presence ) 59 lane width 60 lane deceleration, storage length 61 lane entry transition shape 62 lane offset 63 Num ber of driveway s per site 64 Driv ew ay sp acing from upstream access connectio n 65 Driv ew ay sp acing from down st r eam access connection Ro ad wa y- Drivew ay Inte rs ection 33 Angle of intersection wi th street : fl at-a n g le ( turn an g le < 90 O ) ; ri g ht-a n g le ( turn an g le 90 O ) ; shar p -a n g le ( turn an g le > 90 O ) 34 Cross slope of st reet and shoulder, considered wi th drivew ay grade 35 Curb threshold treat me nt (rolled, vertical lip, counterslope, continuous ) 36 Curb-ter mi nation treat me nt (abrupt end, drop-dow n, returned) 37 Entry transition shap e ( e.g., radius, flare/taper, straight) 38 Entry transition-shape dim ensions (radius, flare dim ensions) 39 Channeliz ation of right turn fr om street into drivew ay 40 Channeliz ation of right turn fr om drivew ay into street 41 Channeliz ation in the driveway (e.g., triangular island to prohibit in and out left-turns) 42 Channeliz ation in st reet - street me dian prohibits all le ft -turns in/out of driveway 43 Channeliz ation in st reet - street me dian prohibits one but not both le ft -turns 44 Drainage: confining the gutter flow 45 Drainage: inlet ty pe and location 46 Clearance from fix ed objects, appurtenance s 47 Pavem ent su rface defor mi ty (corrugation, potholes )

other elements that will affect the user’s position and path when crossing, entering, or exiting the driveway. Means to improve conspicuity include the following: • Clearly defining the edges of shapes, so as to differentiate between shapes (e.g., the edge between a sidewalk and a driveway); • Providing contrast between light and dark surfaces; • Placing a business sign near the driveway, to reinforce its location; and • Installing artificial illumination. Exhibits 5-4 and 5-5 show undesirable design practices. Exhibit 5-4 shows how a planter and a utility pole near the intersection of a driveway with a roadway restrict the sight distance. Visual Exhibit 5-3. Driveway considerations generally outside the control of the designer. Shared Elements, Surroundings 1 Land us e 2 User and vehicle mi x and co mp os ition 3 Te mp oral variation: seas on, da y of week , tim e of da y 4 W eather and weather ef fe ct s Side wa lk -D riv ew ay Intersectio n 5 Sidewalk plac em en t ( adj ac ent to or o ffs et fr om the cu rb or edge) Roadw ay -Dri ve wa y Intersec tion 6 Elevation dif fe rence betw een roadway surface and abutting property Roadway in Vicinity of the Driveway 7 Wi dth of roadway 8 Lanes (num ber, width) 9 Lane ty pe (tra ve l, HOV, bic yc le, turn, par ki ng) 10 Cross slop e ( travel lanes, shoulders ) 11 Hori zo ntal alignm ent of roadway 12 Vertic al prof ile of roadway 13 Sight distanc e re st riction s User Characteristics - Bicyclist 14 Bic yc lis t perception-r eac tion proc ess, tim e 15 Spee d 16 Brak ing capabilit y 17 Sight distanc e nee d User Characteristics - Pedestrian 18 Pedestrian perc eption-reac tion process, ti me 19 Spee d 20 Sight distanc e nee d Spec ial needs groups 21 General - children, elderly 22 Those with disabilities (e.g., mobility, visual) 23 Legal mandates - those related to disabilities User Characteristics - Vehicle, Dri ve r 24 Driv er perception-reaction process, ti me 25 Spee d 26 Deceleration characteristics (ty pical) 27 Brak ing capabilit y (lim iting) 28 Sight distanc e nee d 29 Vehicle widt h 30 Vehicle length 31 Vehicle turning radius 32 Vehicle fr ont overhang, wheelbase, rear overhang, and ground clearan ce di me nsions Geometric Design Elements 27

Exhibit 5-6. Visibility design concerns. Concern or Issue Design Response Specific Procedure and/or Information At driveway intersections with public roadways, have unobstructed lines-of- sight that provide adequate stopping sight distance. Along high-type public roads, adequate intersection sight distance also should be provided. However, this may not always be practical in built-up areas. Bicyclists, motorists in vehicles, and pedestrians need to see each other far enough in advance to avoid collisions. Do not place anything in the border that blocks needed sight lines. Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed stopping sight distance or intersection sight distance. To have time to react, drivers need to detect the driveway well in advance and be able to visually define its shape before entering or exiting. Have driveway edge color contrast with the color of the abutting surface. Have driveway pavement color contrast with the color of the roadway. Consider illumination during darkness. For non-residential, place a monument sign very close to the driveway intersection with the roadway. Curbed driveways provide a clearer delineation of the driveway entry shape than “dust pan” designs do. 28 Guide for the Geometric Design of Driveways obstructions may also make it difficult for motorists on the roadway approaching the driveway connection to have an adequate preview of the driveway or vehicles in the driveway. In Exhibit 5-5, the driver’s view from the street side provides clear definition of the edge. However, for the driver in the parking lot, the curb edge drop off is hidden, so, unable to detect the dropoff, some vehicles drive over the curb. Practices similar to that shown in this exhibit, which create a continuous expanse of pavement and no distinction between the actual driveway and the curb dropoff, should be avoided. A similar problem can occur when a driveway that slopes downward from the roadway edge is located on the high side of a superelevated roadway. Drivers in the roadway attempting to enter the driveway may have difficulty determining where the driveway edges are. A designer may con- sider adjusting the driveway profile so that it rises slightly before descending, or adding delin- eators or soft landscaping to help drivers identify the driveway edges. Exhibit 5-6 offers guidance for sight distance and conspicuity elements. Exhibit 5-4. Placement of planter and utility pole limit the sight distance for a driveway exit. Exhibit 5-5. Poorly defined edge leads to scrapes.

Driveways serving parking garages sometimes have restricted sight distance, especially where the vehicles exiting the garage cross the sidewalk abutting the edge of the garage structure. For guidance, refer to the AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities (5-2). Future studies could provide a better understanding of this. Bicyclists The driveway designer should recognize and accommodate interactions involving bicyclists, motor vehicles, and pedestrians. In this context, the main area of interaction involving bicy- clists is where they cross driveways. This may occur either when a bicyclist is riding in the public roadway, crossing a driveway intersection, or on a bike path or other separate facility that crosses a driveway. Although bicyclists often are not allowed to ride on sidewalks, there are exceptions. In some communities, younger children are allowed to ride on the sidewalk, except in the downtown area (5-3). In some suburban and exurban areas, shared-use sidepaths are a common feature along arterial roadways. In general, bicyclists are more likely to be a consideration at driveways in urban and suburban areas than they are in rural areas. Exhibit 5-7 lists some pertinent design principles. Pedestrians and Pedestrians with Disabilities In many environments, especially in built-up areas, pedestrians will be either crossing the driveway or walking parallel to the driveway. Therefore, pedestrian needs must be considered when designing a driveway. In some environments, pedestrian volumes will be practically nil, and in these situations pedestrian considerations may have less effect on design choices. Where either existing sidewalks cross or future sidewalks will cross driveways, the driveway designer must consider the horizontal alignment, the vertical alignment, and the cross slope of the pedestrian path. In the crossing area, the sidewalk design must conform to ADA requirements. Some sidewalk locations and some sidewalk and driveway designs more easily conform to ADA requirements than do others. Exhibit 5-8 lists some pedestrian design considerations. Geometric Design Elements 29 Exhibit 5-7. Driveway design considerations related to bicyclists. Concern or Issue Design Response Specific Procedure and/or Information Provide horizontal and vertical alignment that provides an adequate advance view of the driveway intersection. Bicyclists, motorists in vehicles, and pedestrians need to see each other far enough in advance to avoid collisions. Do not place anything in the border that blocks needed sight lines. Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed stopping sight distance or intersection sight distance. Abrupt change in cross slope causes bicyclists to lose balance. Where a bicycle path or other similar route crosses a driveway, do not have an abrupt change where the bike path cross slope meets the driveway grade. Abrupt change in surface elevation causes bicyclists to lose control. Where a bicyclist could turn into or turn out of a driveway, do not have an abrupt change in surface elevation that creates a bump for the bicyclist. Relatively thin bicycle tires are vulnerable to openings in the surface. Do not have any grate openings that a bicycle tire could drop into.

30 Guide for the Geometric Design of Driveways Exhibit 5-9 shows methods of aligning sidewalks at driveway crossings, so that the sidewalk does not exceed the ADA 2% cross slope requirement. (Some of these designs could just as easily have a radius return instead of a flared return.) • With the setback or recessed sidewalk location, the driveway rises to the sidewalk elevation over the distance of the sidewalk setback from the curb. • With the ramp or dipped sidewalk, the elevation of the sidewalk drops near the driveway cross- ing. The slope of the ramp on each side of the driveway is not to exceed 8%. • With a sidewalk of sufficient width, a dustpan taper can be constructed adjacent to the curb, and still leave an adequate pedestrian route along the back edge of the sidewalk. • The offset sidewalk is an adaptation of the wide sidewalk, with the sidewalk widened in the vicinity of the driveway to provide enough width for a dustpan and a pedestrian crossing. The accompanying photo shows such an offset or jog in the sidewalk alignment, created so the pedestrian path will not have an abrupt change in elevation. If the normal sidewalk position had been recessed or set back from the curb, with a grass strip between the sidewalk and the curb, then the pedestrian path could continue straight across the driveway, without the jog. Exhibit 5-10 shows an unacceptable treatment. Exhibit 5-11 shows a driveway with very limited sight distance intersecting a sidewalk and a street. A pedestrian crossing a driveway may be affected by factors such as the width of the driveway to be crossed, the volume and the speed of vehicles using the driveway, the design of the side- walk crossing the driveway, the presence or absence of a pedestrian refuge island, or the presence and location of a transit stop or other destination near the driveway. A wider driveway increases a pedestrian’s time of exposure to conflicts with driveway vehicles. The width of the driveway crossing may be more of an issue for a child or older pedestrian who walks slowly than for a wheelchair user. A wider driveway may be more likely to seriously disorient a pedestrian with impaired vision. If pedestrians with impaired vision veer or are misaligned when they cross a driveway, unless Exhibit 5-8. Driveway design considerations related to pedestrians. Concern or Issue Design Response Specific Procedure and/or Information Provide horizontal and vertical alignment that provides an adequate advance view of the driveway intersection. Bicyclists, motorists in vehicles, and pedestrians need to see each other far enough in advance to avoid collisions. Do not place anything in the border that blocks needed sight lines. Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed stopping sight distance or intersection sight distance. Also check sight for people of lower height, such as children or wheelchair users. An excessive sidewalk cross slope (or driveway grade) adversely affects the crossing by pedestrians with vision impairments and those in wheelchairs. ADA requirements specify a pedestrian travel path (called a Pedestrian Access Route or “PAR” in the Draft Public Rights-of-Way Accessibility Guidelines) with a cross slope that does not exceed 2%. The PAR requirement applies not only to the crossing of the driveway, but also to the sidewalk connections. The combination of 2% maximum cross slope and the different sidewalk location options affect the vertical alignment of the driveway in different ways. Is there a suitable pedestrian route across the driveway? The sidewalk alignment across the driveway should be straight and not have “steps” or other abrupt changes in vertical elevation.

Exhibit 5-9. Sidewalk-driveway treatments. Geometric Design Elements 31 Exhibit 5-10. Unacceptable vertical curb where sidewalk crosses driveway. Exhibit 5-11. Driveway with very limited sight distance.

Exhibit 5-12. Bus stop locations near driveways. 32 Guide for the Geometric Design of Driveways another cue such as parallel traffic, a slope, or a guide strip is present, their veer can be expected to be relatively constant during that crossing (5-4). As a result, if someone crossing a driveway initially has a 10-degree bearing error, they will likely continue in that direction. The wider the driveway, the greater the chances are that pedestrians are farther from the sidewalk when they reach the far side of the driveway. On a 20-foot crossing, a pedestrian with a slight veer might be just outside the sidewalk area, and be able to easily locate the sidewalk by reaching out with a cane. On a 30- or 40-foot crossing with the same veer angle, a pedestrian may no longer be able to easily locate the sidewalk on the far side of the driveway area. A cut-through median, a textured pedes- trian crossing, or a delineating guide strip across the driveway width might mitigate this situation on a wide driveway. Guidance strips are sometimes installed in the sidewalk to help guide pedes- trians with impaired vision across wide driveways. However, there is little current research on the ability of pedestrians with impaired vision to use these guidance strips effectively (5-5). Drivers may be more likely to yield to pedestrians if there is a wide landscape strip between the curbline and the sidewalk or an auxiliary deceleration lane, so the vehicle turning into the drive- way can stop outside of the main travel lane of the roadway. For pedestrians with impaired vision, it can be helpful if the driveway design discourages vehicle encroachment onto the sidewalk area. Also, identifying the appropriate time to cross the driveway can be a problem at a busy driveway—this problem may not be amenable to a geomet- ric remedy, except one that discourages high vehicular speeds. Public Transit Facilities The driveway designer should consider the location of transit routes and stops in the vicinity of the driveway. The following problems can arise when driveways and transit stops are too close to each other: • A stopped transit vehicle blocks the driveway. • Transit patrons block the driveway. • Standing transit patrons are uncomfortably close to driveway traffic. • Standing transit patrons block drivers’ lines-of-sight. Exhibit 5-12 illustrates that a bus stop should be located to avoid blocking a driveway and set back a sufficient distance from the driveway to help ensure adequate sight distance. In many cases, a transit stop on the far side of the driveway connection with the roadway is preferable to one on the near side, because far-side bus stops do not interfere with vehicles turning right into driveways and do not block the line of sight to the left of motorists exiting the driveway. When possible, bus stops or driveways may need to be relocated to reduce conflicts with each other. Exhibit 5-13 provides guidance on the location and design of bus stops near driveways. Details on bus stop location and design can be found in TCRP Report 19: Guidelines for the Location and Design of Bus Stops (5-6).

Driveway Plan and Cross-Section Elements The following sections discuss driveway plan and cross-section elements, such as the type of entry/ exit geometry, the amount of flare or radius, the driveway width, and driveway channelization. Driveway Width, Number of Lanes, and Connection Transition This section discusses and presents recommendations for three plan view elements: 1. The normal width of the driveway throat, which does not include the normally found widen- ing or transition with a radius or a flare near the driveway intersection with the roadway; 2. The number of driveway lanes needed; and 3. The shape and dimensions of the shape at the connection (throat entry/exit) transition. The driveway width and the driveway connection transition are separate elements, but the design of each can affect the design of the other, so the discussion of these elements has been combined. Objectives for designing the driveway entry and exit geometry include the following: 1. Define the edge so it is visible for bicyclists, drivers, and pedestrians. 2. Minimize the width of the driveway that bicyclists and pedestrians will need to cross. 3. Design a shape that conforms to the path of the turning vehicle, which enables vehicles to enter a driveway without encroaching into other lanes. 4. Design to enable vehicles to enter the driveway without significantly impeding the upstream flow of through traffic on the roadway. 5. Provide adequate driveway capacity, including providing separate right- and left-turning exit movements, when needed. 6. Design for easy construction. Exhibit 5-14 raises questions that address the design of the connection transition, and Exhibit 5-15 shows the effects of inadequate geometry in this area. The geometry should not force normal right entry or exit movements to cross over the driveway curb or edge, drive on Geometric Design Elements 33 Exhibit 5-13. Driveway design considerations related to nearby bus stops. Concern or Issue Design Response Specific Procedure and/or Information Provide adequate stopping sight distance or intersection sight distance. Provide separation from driveway edge to bus stop. Do not block drivers’ lines-of-sight Do not place bus shelters in a location that blocks needed sight lines. Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed stopping sight distance or intersection sight distance. Length of city transit bus: typical bus, about 40 ft. articulated bus, about 60 ft Space so that the bus does not physically block the driveway Provide distance from driveway edge to bus stop Additional length along roadway needed for maneuvering: where buses do not change lanes: 20 ft where a lane change is required: 50 ft Patrons loading or unloading from transit vehicle do not occupy the driveway Provide a pedestrian standing area separate from and removed from the driveway Connect the bus loading area to the sidewalk Pedestrian connection at least 5 feet wide, where possible

Exhibit 5-14. Design issues for a vehicle turning right into or from a driveway. sidewalk Does a vehicle turning into the driveway encroach into the adjacent lane? Does vehicle encroach to the adjacent lane? roadway lk driveway Does vehicle encroach on the curb or sidewalk? Does vehicle encroach into the adjacent lane? Does vehicle encroach on the curb or sidewalk? Does a vehicle turning out of the driveway encroach into the adjacent lane? Exhibit 5-15. Effects of inadequate driveway radii. (a) (b) (c) 34 Guide for the Geometric Design of Driveways

Geometric Design Elements 35 the sidewalk, or swing wide so that the left side encroaches into adjacent lanes. But excessive width unnecessarily increases the distance across the driveway that bicyclists and pedestrians must cross. Interrelated Factors Affecting the Design Various factors act in concert as a vehicle turns into or out of a driveway and as the vehicle crosses a bicycle lane or sidewalk parallel to the roadway. The following interrelated elements come into play as a driver turns into or out of a driveway intersection (5-7): 1. Visibility and conspicuity of the features that shape the driveway (e.g., opening, edges, markings); 2. Vehicle turning radius; 3. Vehicle tracking width and offtracking characteristics; 4. Intersection-corner treatment and treatment dimensions (e.g., radius or taper dimensions); 5. Width of the lane from which the turn is made (includes offsets, edge flares); 6. Width of the lane into which the turn is made (includes offsets, edge flares); 7. Angle of the intersection; 8. Cross slope of the pavement surface in the turn; 9. Pavement surface condition (e.g., in extreme cases, a corrugated surface or pothole can impart vertical acceleration to a turning vehicle, decreasing the available side friction); 10. Turning speed; 11. Driver’s tolerance of lateral acceleration; and 12. Driver’s ability to perceive these elements. The vertical profile also affects the driving experience. Before selecting the dimensions of the connection transition, the designer should identify design vehicles for the particular driveway. Chapter 3 includes a discussion of design vehicle con- siderations. Where heavy vehicles may run over area behind the curb and damage surfaces such as a sidewalk or a driveway median, the designer should consider a strengthened pavement sur- face for the affected area behind the curb. Number of Lanes A basic driveway design question is “how many driveway lanes should be provided?” Typically, driveways serving a single-family residence are one or two lanes wide, often reflecting the width of the garage. Driveways serving farms and fields are typically one lane wide, although the width of that lane is quite wide, reflecting the widths of farm machinery. In general, driveways serving commercial and industrial sites should have at least two lanes (one-way driveways are an obvious exception), operating with one lane in each direction. With increasing driveway volume, adding a second exit lane becomes highly desirable in order to avoid excessively long queues and delay. Without two exit lanes, a motorist waiting for gaps in both traffic directions before turning left out of the driveway will unnecessarily block other motorists in the exit queue who could otherwise turn right when there are gaps in the traffic from the left. However, the number of lanes exiting from the development and turning in one direc- tion must not exceed the number of available traffic lanes on the roadway in that direction. For example, for a driveway entering a two-lane two-way roadway, no more than one lane in each direction (a total of two exit lanes) should be allowed to exit the driveway. Generally, dual exit- lane driveways are desirable when the exit volume reaches the level that more than one vehicle will want to exit the driveway within the time interval it takes one left-exiting vehicle to wait for and accept an adequate gap in roadway traffic, or when the driveway intersection with the pub- lic road is signalized. Exhibit 5-16 shows some of the more common commercial driveway con- figurations, excluding those for very high volumes.

Exhibit 5-17. Undesirable wide-open driveway. Exhibit 5-16. Common choices for a range of commercial driveway designs. Width for 1 entry and 2 exit lanes; separated by median Width for 2 lanes. (Without a marked center line, there may be more encroachments.) Width for 1 entry and 2 exit lanes; separated by yellow markings yellow white white 36 Guide for the Geometric Design of Driveways If the driveway forms the fourth leg of an intersection, additional lanes may be needed. In such cases, a configuration of three exit lanes (left turn, through, and right turn) and/or two entry lanes may be desirable. At what might be considered the high-volume end of the spectrum, sites such as major shopping centers and urban activity centers, even more lanes may be needed. If there is a question of whether additional lanes are needed, an operational analysis of the inter- section between the driveway and the roadway, perhaps using calculations from the Highway Capacity Manual (5-8), could be performed. Variables reflected in the operational analysis include the volume on the main roadway, the volume and directional distribution of the driveway traffic, the number of adequate size gaps in through street traffic, and the form of traffic control at the driveway/roadway intersection. Driveway Width The width of a driveway is its normal width, measured some distance back from its intersection with the roadway. It is not the width that includes widening near the intersecting roadway. The width of a driveway is a function of the number of driveway lanes, the widths of those lanes, and the presence and width of a median. The width of a driveway should reflect the needs of both vehicular and pedestrian traffic. The competing goals of reducing vehicle delay by adding lanes and reducing pavement width to facil- itate pedestrian crossings need to be recognized. Exhibit 5-17 shows a wide-open, undefined driveway across what appears to be the full frontage of the tract. These designs are particularly unfriendly to bicyclists and pedestrians crossing the excessive driveway opening width. Because of the lack of lane definition, vehicles enter and leave such sites in random positions and are more likely to cross paths. Such a design should be avoided.

Exhibit 5-18 presents ranges of driveway widths and radii given in response to a survey of transportation agencies. Connection Transition Shape For a driveway to intersect a public roadway, a break in the curb line or the edge of the road- way is required. This section discusses aspects of the transition that occur past the break, within the first few feet of the driveway. This transition may be designed with a perpendicular edge, rec- tangular apron, flare or taper, or curved radius. Exhibit 5-19 shows basic types of driveway con- nection transition geometries. The driveway connection transition, where turning vehicles enter and exit the driveway, is usu- ally a critical design area, given that it is the location of potential interaction of entry and exit movements (5-9). One of the key aspects of good driveway design is accommodating the entry and exit movements so that they do not encroach on one another or vehicles in other lanes (5-9). In some situations, this requires that the driveway radius be almost as large as the turning radius of the selected design vehicle. Except on low-volume, low-speed roadways, the curb radius or flare dimensions should be designed so that normal right-turn entry movements do not have to slow down to a near stop in the through travel lanes on the roadway. The dimensions should also allow drivers to turn into or from a driveway without encroaching into conflicting lanes of traffic. Where the roadway is curbed, the entry shape also acts in concert with the curb termination treatment at a driveway entry. Curbs may be terminated abruptly, by means of a drop-down curb, or by a return curb. Exhibit 5-20 shows curb termination treatments. (When the drop-down curb design accompanies a flare/taper transition edge shape, this is sometimes called a “dust pan.”) Exhibit 5-21 compares the connection transition shape alternatives. The perpendicular edge is the easiest to construct, but the least conspicuous to both motorists and pedestrians and does not conform to or help define the path of a turning vehicle. The rectangular apron is better than Geometric Design Elements 37 Exhibit 5-18. Range of reported driveway widths and radii. . Normally, Use This in Most Situations (ft) Commercial (ft) Residential (ft) Sm al le st re po rte d Av er ag e La rg es t re po rte d Sm al le st re po rte d Av er ag e La rg es t re po rte d Sm al le st re po rte d Av er ag e La rg es t re po rte d Width for 2-way: normal maximum (ft.) 24 34 40 35 40 46 12 23 30 Width for 2-way: normal minimum (ft.) 12 24 35 12 22 30 8 12 15 Entry-shape plan-view dimensions for curved radius, maximum R (ft.) = 20 41 75 40 50 70 10 23 35 for curved radius, minimum R (ft.) = 3 16 25 15 21 30 3 11 15 NOTE: These values reflect survey responses from 1 local and 16 state transportation agencies. Exhibit 5-19. Types of driveway connection transition geometry. Curved radius drivewaydriveway drivewaydriveway THESE ARE PLAN (TOP) VIEW DRAWINGS back of curb back of curb back of curb back of curb roadway Flare/Taper roadway Rectangular apron roadway Perpendicular edge roadway

Exhibit 5-20. Methods to terminate the curb. roadway roadway curb curb driveway Method to terminate the curb: RETURN CURB driveway roadway curb Method to terminate the curb: DROP-DOWN CURB driveway roadway curb Method to terminate the curb: ABRUPT END driveway (a) (b) (c) Exhibit 5-21. Comparison of connection transition shape alternatives. Design Objectives Perpendicular Edge Rectangular Apron Flare/Taper Curved Radius Conforms to path of turning vehicle worst (1) poor (2) better (3) best (4) Definition of edge for motorists poor (2) worst (1) better (3) best (4) Definition of edge for pedestrians best (4) worst (1) poor (2) better (3) Ease of construction best (4) better (3) better (2) worst (1) Overall score best (12) NOTE: Cannot compare scores directly, because the importance or weight of each objective is not equal. 38 Guide for the Geometric Design of Driveways the perpendicular edge in terms of functionality, but slightly more difficult to construct. The use of both types should be limited to single-family or duplex residential units. The flared taper is easier to build than the curved radius, but is less effective in terms of conspicuity and conform- ing to the path of a turning vehicle. Therefore, the use of the flared taper generally should be lim- ited to low intensity or medium intensity uses. It has been stated that “Flared driveways are preferred because they are distinct from intersection delineations . . . “ (5-1, p.398); in other words, because they do not look like roadway intersections, motorists can distinguish between driveways and side streets. While this may be a benefit in a few situations, in many situations there is no benefit to be had from this distinction, and even if there were, other aspects of driveway design will provide a visual difference for motorists to rely on. As for curb termination treatments, an abrupt end is more likely to snag a vehicle tire, and therefore is undesirable. A returned curb has a vertical face, which provides entry-edge definition for an approaching motorist. A discussion of design treatments for sidewalks crossing the driveway in this area is in Pedestrians and Pedestrians with Disabilities. Exhibit 5-22 presents a table from the 2005 Florida Driveway Handbook (5-9, p.31), which was derived from much older sources. The numerical values illustrate the inverse relationship between entry radius and the width of the entry lane: as the size of the radius increases, less entry lane width is needed. Based on recent experience, these dimensions may be generous for many drivers of passenger cars. Connection Transition Design Suggestions The preceding discussion of driveway transition shapes leads to the suggestions in Exhibit 5-23. Driveway Width Design Suggestions When establishing driveway widths and driveway opening treatment dimensions (e.g., size of radius or flare), it is not uncommon to encounter opposing viewpoints; there may be conflict- ing objectives between the various driveway users, such as pedestrians and motorists, with some

Geometric Design Elements 39 Exhibit 5-22. Inverse relationship between entry radius and entry lane width. Exhibit 5-23. Driveway transition shape design guidelines. Category Description of Common Applications* Suggested Driveway Transition Shape Design (assuming curbed roadways in urban area, uncurbed in rural area) STANDARD DRIVEWAYS Very high intensity Urban activity center, with almost constant driveway use during hours of operation. Typified by a driveway serving a post-1950 major shopping center or office complex. Not uncommon for such driveways to be signalized. Design as a street intersection. Provide separate right- and left-turn lanes on approaches to public roadways. Higher intensity Medium-size office or retail, such as community shopping center, with frequent driveway use during hours of operation. Also includes land uses with extreme peaking patterns, such as public schools, worship assemblies, and employee parking lots. Use curb radius design. Consider separate right- and left-turn lanes on approaches to public roads. Medium intensity Smaller office or retail, such as convenience stores, with occasional driveway use during hours of operation. Also includes some apartment complexes. Curb radius design is preferred. Lower intensity Typical applications include single-family or duplex residential, other types with low use. May not apply to rural residential. Use either the curb radius or the flare/taper design. SPECIAL SITUATION DRIVEWAYS Central business district Building faces are close to the street. May have on-street parking or bus stops, a continuous sidewalk from the curb to faces of buildings, and higher pedestrian usage than in most other environments. Many situations will serve P-cars and some single-unit trucks. Design will vary depending on location, land use, and traffic volumes. Farm or ranch May be a mixture of residential and industrial characteristics, used by a mix of design vehicles, such as P-car, single-unit truck, and agricultural equipment. Design uncurbed radius or taper to accommodate farm/ranch vehicles. Field Serves a field or other similar rural land area that is seldom trafficked. Higher-clearance P- vehicles or heavy vehicles are expected. Design uncurbed radius or taper to accommodate farm/ranch vehicles. Industrial Driveways frequently used by buses, tractor with semi-trailers, and other vehicles longer and wider than the design passenger car. Design for trucks. The driveway may need a special design to accommodate the extra axles and longer wheelbase that will lead to much greater offtracking of vehicles entering the driveway. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.

calling for smaller dimensions to make crossing the driveway easier for pedestrians, and others wanting larger dimensions to facilitate motor vehicle ingress and egress. An operational analysis of the intersection between the driveway and the roadway provides a basis for decisions regarding the number of driveway lanes. The connection transition and the driveway width dimensions should complement each other to produce good driveway opera- tions. The driveway width and the curb radius can perform in concert, so to some degree one can increase as the other decreases. In other words, a wide driveway can be used together with a small radius or flare to achieve similar operations to a narrower driveway with a larger radius or flare. When only one vehicle is expected to be using the driveway at any given time, such as a res- idential driveway serving a two-car garage, the smaller radii are suitable with the greater widths. Exhibit 5-24 offers guidelines for driveway width and radius. These dimensions do not con- sider the presence of an offset between the outer edge of the traveled way and the end of the Exhibit 5-24. Driveway width and curb radius guidelines. Category Description of Common Applications (Note: These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.) Driveway Width Driveway Curb Radius (in ft) Higher speed road Moderate speed road Lower speed road STANDARD DRIVEWAYS Very high intensity Urban activity center, with almost constant driveway use during hours of operation. Many justify two lanes in, two to three lanes out. Refer to street design guides. 30–50 25–40 NA Higher intensity Medium-size office or retail (e.g., community shopping center) with frequent driveway use during hours of operation. One entry lane, 12–13 ft wide Two exit lanes, 11–13 ft wide. 25–40 20–35 NA Medium intensity Smaller office or retail, with occasional driveway use during hours of operation. Seldom more than one exiting vehicle at any time. Two lanes, 24–26 ft total width 20–35 15–30 NA Lower intensity Single-family or duplex residential, other types with low use, on lower speed/volume A mix of design vehicles; some may be very low volume. roadways. May not apply to rural residential. May be related to the width of the garage, or driveway parking. Single lane: 9–12 ft Double: 16–20 ft 15–25 10–15 5–10 SPECIAL SITUATION DRIVEWAYS Central business district Building faces are close to the street. Varies greatly, depending on use NA 20–25 10–15 Farm or ranch; Field Min. 16 ft, desirable 20 ft. Affected by widths of field machinery. 30–40 20–30 NA Industrial Driveways are often used by large vehicles. Minimum 26 ft 50–75 40–60 40–60 NOTES: These widths do not include space for a median or a parallel bike lane or sidewalk. Additional width may be needed if the driveway has a curved horizontal alignment. For a flare/taper design, use the radius as the dimension of the triangular legs. For industrial or other driveways frequented by heavy vehicles, consider either a simple curve with a taper or a 3-centered curve design. For connection angles greatly different than 90 degrees, check the radius design with turning templates. For connection corners at which a turn is prohibited, a very small radius is appropriate. Also see the section, Driveway Horizontal Alignment and Angle. Driveways crossing an open ditch should have a minimum 2 ft shoulder on each side. (source: Statewide Urban Design and Specifications, Iowa State U., Ames, IA (October 21, 2008) p. 4. If the roadway has a usable shoulder, a somewhat smaller radius may perform acceptably. 40 Guide for the Geometric Design of Driveways

Geometric Design Elements 41 driveway, i.e., the driveway threshold. There are arguments for and against adjusting the radius when an offset is present. Some agencies reduce the required radius when an offset is present, expecting turning vehicles to follow an effective radius that utilizes the space between the outer edge of the traveled way and the threshold. Arguments against this practice include an assump- tion that some drivers may not follow the imaginary effective radius, but instead try to follow the visible physical connection transition edges. Also, it is possible that, in future, the roadway cross section width may be reallocated and the offset eliminated, resulting in an undersized connection transition. One-Way Driveway Widths Only a small fraction of driveways operate in a one-way mode. Information on which to base guidance for the design of one-way driveways is limited and, as Exhibit 5-25 shows, current agen- cies’ standards differ considerably. Structured studies of one-way driveway design elements would be helpful. Throat Transition Design for Larger Vehicles The offtracking of even-turning single-unit trucks can result in tires running over the curb or the sidewalk behind the curb. But if the designer accommodates turning trucks with a sim- ple radius design, this accommodation may create a very wide entry opening. To better accom- modate the wheel paths of turning trucks without paving such a wide area, refer to the AASHTO Green Book’s (5-1, p. 583–621 ) discussion of designing simple curves with a taper and design- ing three-centered compound curves. Exhibit 5-26 dissects the geometry of a three-centered curve at a 90-deg. intersection. Throat Width for Curved Driveways If the driveway horizontal alignment is curved instead of straight, then additional driveway width may be required to account for the effects of vehicle offtracking. Refer to the AASHTO Green Book (5-1, p. 202–223) for procedures to determine how much additional width is needed. Throat Transition Widening Some driveways are constructed with a wider section close to the intersecting roadway, then the width tapers to a narrower section some distance back from the intersecting roadway. Two of the reasons for doing this are to widen the driveway 1. To provide additional lanes at the intersection with the public roadway; and 2. To accommodate the offtracking and swept paths of turning vehicles entering and exiting driveways at the entry/exit area. In either case, the designer will need to design a transition from the wider cross-section width to the narrower cross-section width. Exhibit 5-27 shows a schematic of this concept. A 6:1 taper would be adequate for an assumed design speed of 19 mph. Tapers of 8:1 to 12:1 should be more than adequate for the typical driveway, excluding those that look and operate like public roadways. Exhibit 5-25. One-way driveway widths from selected states. Agency Source Category Width for one-way Florida Driveway Handbook Urban 12 ft. minimum Missouri 940.16 (5/13/09) Driveway 20–30 ft New Jersey C-11 (6/20/07) Driveway 20–23 ft New York 608-03 (1/8/09) Minor Commercial 12–24 ft; 16 ft normal Utah 12.1.1601.10 Driveway 12–32 ft Exhibit 5-26. Geometry of a sym- metrical 3-centered curve. Offsetfset R 1 R 2 R2 R1

Exhibit 5-28 shows an example 12:1 taper design. For a driveway having 12-ft-wide lanes, the 15-ft radius provides a motorist with a 13.9 ft (12.0 + 1.9) opening at the point of tangency (PT), and the 20-ft radius produces a 13.5 ft (12.0 + 1.5) opening at the PT. Channelization Various types of channelization are sometimes incorporated into driveway designs. These include medians in the driveway, islands in the driveway, and channelization in the roadway at the intersection with the driveway. The general design objectives for channelization are to • Separate conflicting movements (including opposing directions of travel) • Control the angle of conflict • Reduce excessive pavement area • Regulate traffic and indicate proper use of driveway/intersection • Provide pedestrian refuges/protection • Provide for protection and storage of turning and crossing vehicles Where channelization is desired but there is not sufficient space to accommodate the width of a median or island, some agencies have installed channelizing devices such as tubular markers. These are discussed later in the section, Traffic Controls. Channelization in the Roadway Medians are sometimes labeled as being either non-restrictive or restrictive. A non-restrictive median is a median or painted centerline that does not provide a physical barrier between center traffic turning lanes or traffic lanes traveling in opposite directions; examples include continuous center turn lanes and undivided highways. Restrictive medians physically separate vehicles travel- ing in opposite directions and restricts the movement of traffic across the median; (e.g., a concrete barrier or guard rail, a raised curb island, or a grassed or swaled median). Either type can be designed to provide some degree of separation between opposing traffic flows and provide space for left- turning vehicles out of the through lane. Restrictive medians offer several safety benefits. Restrictive medians in the roadway provide refuge for pedestrians crossing the roadway. Some median openings allow all traffic movements to be made. Other openings restrict left-turn and other movements across the median. Prohibiting movements translates into fewer conflicts, greater safety, and more uniform travel speeds along the arterial. However, these benefits may be somewhat offset by the increased turning volumes where there are full-movement median openings. Restrictive medians are often used because, as Exhibit 5-28. Example taper design. X = 36’ Qdriveway 12:1 taper Y = 3’ Offset R TA N TAN 2 R PC PT ro ad wa y Taper angle 4.764O 4.764O / 2 42.618O 42.618O R 15 ft 20 ft TAN 13.80 ft 18.40 ft Q 13.75 ft 18.34 ft Offset at PT 1.85 ft 1.47 ft 42 Guide for the Geometric Design of Driveways Exhibit 5-27. Taper to effect throat transition widening.

Geometric Design Elements 43 indicated in Exhibit 5-29, more than two thirds of driveway crashes are related to left-turn entry or exit movements (5-10). Exhibit 5-30 shows how restrictive medians may be used to eliminate some or all of the left- turn movements in and out of driveways. They may be channelized to allow for left-turns from the roadway, while prohibiting left-turns out. Alternatively, they may be channelized to allow for left-turns from the driveway, while prohibiting left-turns in. The following items are aspects of good median approach-island design (5-11): • The approach nose should be offset from the approach lanes to minimize accidental impacts. • The shape of the island should be based on the turning path of the design vehicle and the island function. • The length of the island should be related to approach speed and reflect available width, taper design, and local constraints. • The width of the island should adequately serve its intended functions (e.g., access control, pedestrian refuge, separation of conflicts, and shielding left-turn lanes). • Median islands should begin on tangent alignment and on upgrades or well past crest vertical curves. It may be appropriate to extend a median island to avoid its introduction on a hori- zontal curve or within an area of limited sight distance. Designs that prohibit some left-turn movements are often accompanied by an analysis of traffic patterns that identifies alternate means of accomplishing a left turn, such as downstream U-turns or other indirect means. U-turns are used as an alternative to direct left turns to reduce conflicts and improve safety along arterial roadways.They make it possible to eliminate left-turn movements into and out of driveways. They also make it possible to eliminate the need for certain traffic signals (or reduce the number Exhibit 5-29. Driveway crash types related to maneuver and orientation. Percent of Total Maneuver______Turn____Collision_______Driveway Crashes Entering Left Rear-end 26 Leaving Left Right-angle 24 Entering Left Head-on angle 15 Entering Right Rear-end 12 Leaving Right Right-angle 7 Leaving Right All other 8 Leaving Left All other 3 Entering Right All other 3 Entering Left All other 2 100 Source: Box, Public Safety Sys., 1969 Exhibit 5-30. Using restrictive medians to eliminate some left-turn movements. Source: Transportation and Land Development, Second Edition.

44 Guide for the Geometric Design of Driveways of signal phases) that would not fit into a progression pattern (5-12). The median openings where U-turns would be made need to be designed to accommodate the additional turning movements. Requests for a median opening or opposition to closing a median opening may be based on an assumption that a direct left-turn egress maneuver is preferable to a right turn followed by a U-turn. However, observations show that drivers often make a right turn followed by a U-turn where the median opening design permits a direct left turn from a driveway (5-13). The addi- tional travel distance of turning right and then making a U-turn is offset by travel time savings by not having to wait for a gap in both directions that is needed for direct, left-turn egress. Medians in the Driveway The benefits of restrictive medians in a roadway can also accrue when medians are installed in driveways. Medians in a driveway may be appropriate where one or more of the following conditions exists: • The driveway has two or more entrance lanes. • The driveway has two or more exit lanes. • There is a large pavement area that may confuse drivers. • The driveway operates as right-in/right-out, and this may be unclear to some drivers. • The driveway serves a high volume of traffic. • The driveway is or will be signalized. A median in a driveway that separates the ingress and egress movements is appropriate for very high-intensity driveways, where the median may provide refuge for pedestrians, separate the opposing traffic flows, and channelize the traffic movements. Exhibit 5-31 provides guidance for when a median in a driveway may be beneficial. The presence of a median will make the overall length of the pedestrian crossing wider. However, this may be more than offset by the pedestrian refuge effect the median creates in the middle of the driveway. Where a driveway median is needed, there are minimum dimensions that apply to avoid hav- ing a median that is too short and narrow. There is also a possibility that if the median island is too wide, drivers may mistake one driveway with a median for two separate driveways (5-9, p. 46). Exhibit 5-31. Driveway median use recommendations. Driveway Category Description of Common Applications (Note: These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.) Applicability of Median in Driveway STANDARD DRIVEWAYS Very high intensity Urban activity center, with almost constant driveway use during hours of operation. Applicable Higher intensity Medium - size office or retail, such as community shopping center, with frequent driveway use during hours of operation. May be applicable Medium intensity Smaller office or retail, some apartment complexes. Usually not applicable, but may be applicable for some wider driveways Lower intensity Single-family or duplex residential, other types with low use. May not apply to rural residential. Not applicable SPECIAL SITUATION DRIVEWAYS Central business district Building faces are close to the street. Usually not applicable, but may be applicable for some wider driveways Farm or ranch; Field A mix of design vehicles; some may be very low volume. Usually not applicable Industrial Driveways are often used by large vehicles. Often not applicable, but may be applicable for some wider driveways

Exhibit 5-32. Driveway median design guidelines. Aspect Suggested Design Rationale Length Minimum 40 ft, preferable 50 ft or more Need adequate length for conspicuity, effectiveness. Width Absolute minimum: 4 ft Minimum to provide pedestrian refuge: 6 ft Width for visibility of landscaping: 8 to 10 ft Maximum for a driveway divisional island (width of the part that is unavailable for travel, i.e., not including turn lane widths): 12 to 16 ft. Absolute minimum based on the Green Book. Maximum based on potential for drivers to mistake one driveway with a median for two separate driveways. End treatment The 2004 AASHTO Green Book states that for a median island less than 10 ft wide, a semicircular end shape is adequate. For median island widths of 10 ft or more, a bullet nose end shape is suggested. From observations, a bullet nose shape may be desirable for widths of less than 10 ft. To fit the wheel path of a turning vehicle, per the 2004 Green Book (p.697). Geometry of a bullet nose median-end shape Radius of nose Median width 2 Stop line Radius of nose Half-bullet nose median-end shape Exhibit 5-32 lists suggested minimum dimensions and presents two versions of bullet nose end geometry. The half bullet nose provides a larger radius to accommodate the path and offtrack- ing of a vehicle nearing the end of its left turn. If the stop line and stopped position for vehicles leaving the site is close to the median nose end, then a lesser radius may be adequate for that move- ment. Examine the turning paths of left-turning vehicles to ascertain what shape will suffice. Islands in the Driveway Driveway triangular islands (pork chops) can be constructed in the driveway entry throat at driveway intersections with both divided and undivided roadways to • Channelize right turns, • Discourage or prohibit one or both left turns, or • Provide refuge for pedestrians. Exhibit 5-33 illustrates three different scenarios for using islands to discourage left turns. A triangular island and an angled driveway can have some design objectives and features in common. The objectives of either design can include facilitating right turns and discouraging left turns. With both, the design can attempt to align vehicles at a skewed angle rather than per- pendicular to the public roadway. Exhibit 5-34 shows two schematics for islands to channelize right turns exiting a driveway (5-13). In the (a) schematic, a flatter entry-angle combined with a larger radius may increase the speed at which right-turning vehicles leave the driveway. The flatter entry-angle requires the driver’s head to turn a greater number of degrees to the left to monitor oncoming traffic from the left. If the design evokes a subconscious association with a freeway entry ramp, it could theoretically give the driver a false sense of a free entry into the through roadway. This arrangement has been criticized for being Geometric Design Elements 45

unfriendly to pedestrians because of the relatively high speed of the right turn and the need for drivers to scan a wide angle for pedestrians. The practice of placing the pedestrian crosswalk in the middle of the curve, however, affords drivers an improved view of pedestrian crossings. The second schematic (b) shows an alternative design that has several advantages. Motorists turn- ing right from a driveway can more easily see approaching through traffic. This design is more pedes- trian friendly because drivers have a better view of the sidewalk and the speeds are relatively slow. The island area should be sufficiently large to command attention. Refer to the AASHTO Green Book (5-1) to find the recommended minimum area and dimensions on a side. At loca- tions where there is a likelihood of traffic, especially large trucks, overrunning the island, there may be a need for a mountable curb and structural pavement within the island. Along roadways lacking a restrictive median, observations of traffic movements at triangular islands indicate it is not uncommon for drivers to make unusual maneuvers to circumvent the island and make prohibited movements. It is challenging to identify a shape, size, radius, and other features that will effectively discourage drivers from circumventing the island. Florida DOT does not use driveway triangular islands on undivided roadways (5-9). Lakewood, Colorado, uses a 40- by 18-ft island (360 sq ft) where all left turns are prohibited and a 20- by 18-ft (180 sq ft) island where only the left turns leaving the driveway are prohibited. A design recommended for South Exhibit 5-33. Using islands to discourage left-turn movements. Source: Transportation and Land Development, Second Edition. Exhibit 5-34. Comparing two right-turn island designs. Source: Transportation and Land Development, Second Edition. 46 Guide for the Geometric Design of Driveways

Geometric Design Elements 47 Dakota incorporated a long stem on the driveway-end of the triangular island to discourage wrong-way movements (see Exhibit 5-35). To better achieve the objective, triangular island installations have been accompanied by the installation of a barrier median or by vertical pylons (traffic posts) along the middle of the public roadway. Inappropriate Channelization Although channelization can be beneficial, it can also be ineffective or inappropriate in some situations. One does not have to look far to find examples (e.g., Exhibit 5-36) of questionable Exhibit 5-35. Design proposed to discourage circumventing triangular island. Source: Dye Mgmt. Group, Review of SDDOT’s Hwy. Access Control Process, Feb. 2000 CHANNELIZING ISLAND roadway dr ive wa y Exhibit 5-36. Channelization with dubious benefits. (a) (b) (c)

Exhibit 5-37. Sidewalk with visual, tactile, and geometric cues crossing a driveway. 48 Guide for the Geometric Design of Driveways island designs. The islands in the photos seem too small. It appears that the island in the upper photo is aligned in a way that forces right-turning vehicles off of the pavement into the dirt. One can guess at the intent for islands in photos (b) and (c), but one wonders if they have any posi- tive effects on traffic, or if they are just obstacles and nuisances. Visual and Tactile Cues Providing visual and tactile cues that distinguish the sidewalk and define it separately from other driveway areas can assist pedestrians having visual impairments to cross the driveway effi- ciently and safely. Texture, visual contrast, and slope differences are desirable. Exhibit 5-37 shows a sidewalk crossing a driveway. The driveway has a distinct slope toward the street. The slope between the street edge and the sidewalk edge is much greater than the slope across the sidewalk. The difference between the slopes may help pedestrians with vision impair- ments distinguish between the two areas and avoid accidentally veering into the street area as they cross the driveway. There is also a color difference between the sidewalk and the driveway throat area and a slight texture difference between the sidewalk and asphalt which can be detected by some pedestrians using a long cane. Except for signalized driveways or a few other cases, the use of detectable warning surfaces, such as truncated domes, is discouraged because overuse of detectable warnings surfaces may make it more difficult for pedestrians with vision impairments to recognize streets and to maintain their orientation (see Exhibit 5-38 for further discussion). Exhibit 5-39 shows a typical driveway con- struction plan for a detectable warning surface on a sidewalk at the edge of a signalized driveway. Driveway Cross Slope Where the driveway intersects the roadway, one side of a driveway will be higher than the other side, unless the roadway that the driveway intersects is perfectly level. Proceeding from the trav- eled edge toward the private property, the designer can alter the relative difference in elevation between the two outer edges of the driveway by having different edge profile grades. Throughout

the length of the driveway, one edge may be higher than the other, or the center line may be higher than the edges, creating driveway cross slope. Where the driveway and sidewalk intersect, the driveway cross slope is the same as the sidewalk grade. In the absence of information specifically developed for driveways, these guidelines have incor- porated cross slope recommendations from AASHTO. A minimum cross slope of 2% is recom- mended to provide surface runoff drainage. Where feasible, a maximum cross slope of 8% is recommended in areas where snow or ice can occur. Geometric Design Elements 49 Exhibit 5-38. When to use detectable warning surfaces. Advisory R221 Detectable Warning Surfaces. Detectable warning surfaces are required where curb ramps, blended transitions, or landings provide a flush pedestrian connection to the street. Sidewalk crossings of residential driveways should not generally be provided with detectable warnings, since the pedestrian right-of-way continues across most driveway aprons and overuse of detectable warning surfaces should be avoided in the interests of message clarity. However, where commercial driveways are provided with traffic control devices or other- wise are permitted to operate like public streets, detectable warnings should be provided at the junction between the pedestrian route and the street. Source: Revised Draft Guidelines for Accessible Public Rights-of-Way, November 23, 2005 [http://www.access-board.gov/prowac/draft.htm#221 ] Exhibit 5-39. Example of a detectable warning surface at edge of signalized driveway.

Driveway Horizontal Alignment and Angle The alignment of a driveway near the connection with the public roadway affects traffic operations and safety on both the driveway and the roadway. This section addresses driveway horizontal alignment and the angle of intersection with the roadway. When there are driveways on opposite sides of the roadway from each other, the designer should check the alignment of through lanes, turning lanes, and medians on both the intersect- ing roadway and the driveway for potential operational and safety problems. The through lanes should not be offset, but aligned. The review should include a check for whether the lane and median locations would create an offset that would obstruct sight distance (e.g., the sight distance between a vehicle turning left and an opposing through vehicle). Angle of Intersection Just as it is undesirable for two roadways to intersect at highly skewed angles, it is undesirable for most driveways to intersect the roadway at a large skew. When a skew angle forces drivers to deal with a turning angle that is much less than or greater than 90 degrees, drivers will have greater difficulty turning their heads to scan the through roadway for an adequate gap, and more distance and time is required to complete an acute angle turning movement. If the crossing is perpendi- cular to the driveway, it will also be more difficult for bicyclists and pedestrians about to cross a driveway to look over their shoulders to spot vehicles turning from the main roadway. Research studies have concluded that the intersection angle should not be skewed from 90 degrees by more than 15 to 20 degrees (5-14 through 5-16). One-way driveways are an exception to this, and they have operated successfully with skew angle intersections with the roadway. Exhibit 5-40 lists minimum allowable angles reported in a survey of transportation agencies. For two-way driveways, the average value allowed no more than about 20-deg. deviation from 90 degrees. Where the one-way driveway is intended to operate in a right-turn entry-only or a right-turn exit-only manner, there are tradeoffs. One theory is that a flatter angle (e.g., 45 degrees) makes it less likely that drivers will violate the right-turn-only intention or use the driveway in the wrong direction. On the other hand, the flat exit requires that drivers entering the roadway turn their heads much more than 90 degrees to see oncoming traffic from the left. Also, the greater the skew angle, the greater the crossing distance parallel to the roadway for bicyclists and pedestrians. A review of state standards indicates that few allow an angle less than 60 degrees at one-way driveways. Given strictly as an example, the following passage and drawing from the Ohio DOT manual regulate the angle at which a driveway intersects the roadway: 803.21 Drive Intersection Angle New drives should intersect the highway at an angle between 70° and 90°. However, in some cases, it may be necessary to retain existing drive angles that vary from these desirable angles. 50 Guide for the Geometric Design of Driveways Exhibit 5-40. Range of reported allowable driveway intersection angles. Normally, Use This in Most Situations Commercial Residential Sm al le st re po rte d Av er ag e La rg es t re po rte d Sm al le st re po rte d Av er ag e La rg es t re po rte d Sm a lle st re po rte d Av er ag e La rg es t r e po rte d For 2-way drive, minimum angle with the roadway allowed (90O is right-angle) 60 68 90 60 69 75 60 70 90 For 1-way drive, minimum angle with the roadway allowed (90O is right-angle) 45 64 90 45 68 90 45 66 90 NOTE: These values reflect survey responses from 1 local and 16 state transportation agencies.

Geometric Design Elements 51 Exhibit 5-41 shows a guideline for angled, one-way driveways, from Ohio DOT design details. Exhibit 5-42 lists suggested minimum allowable intersection angles for driveways. Driveway Horizontal Alignment Past or back from the driveway connection transition area (the intersection with the roadway), the horizontal alignment (i.e., plan view) of a driveway should be straight, not curved. One rea- son for this is so the driver of a motor vehicle entering or leaving the driveway does not have the added task of steering in a compound or reverse or multiple curves, which diverts more atten- tion from the task of monitoring crossing bicyclists, pedestrians, and vehicles. Another reason is that a straight alignment makes it easier for drivers to position and align their vehicles as they approach the intersection and make turning maneuvers and not sideswipe other vehicles. A third reason is to avoid creating situations where the vehicle exiting the site is unintentionally posi- tioned at a skew angle to the roadway. Exhibit 5-43 recommends minimum lengths of straight approaches in advance of the actual physical intersection of a driveway with a roadway. Space for Bicyclists and Pedestrians Motor vehicles are not the only form of traffic traveling perpendicular to the roadway to and from a traffic generator set back from the roadway. Bicyclists and pedestrians also make these movements at many locations and, in the absence of a separate facility, they may bike or walk in the driveway. Exhibit 5-44 shows a pedestrian on a gray, overcast day with light rain, forced to walk in the driveway because of the lack of a sidewalk. At this particular location, the lack of a sidewalk con- tributes to occasional conflicts between vehicles and pedestrians. Exhibit 5-41. Example of a skew-angle driveway. Exhibit 5-42. Suggested driveway intersection angles with roadway. Driveway Category Description of Common Applications* Minimum Allowable Driveway Intersection Angle in Degrees STANDARD DRIVEWAYS Very high intensity, Higher intensity, Medium intensity 70 Lower intensity Very infrequent use, such as single-family or duplex residential, on urban lower volume, lower speed roadways 60 SPECIAL SITUATION DRIVEWAYS CBD, Farm or ranch, Field, Industrial 70 One-way, for either right-turn entry-only or right-turn exit-only Flat, acute angle may discourage wrong-way use 45 to 60 * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.

Exhibit 5-45 suggests situations where a separate facility parallel to the driveway may be needed and where it may be acceptable for the bicyclist or pedestrian to share the driveway with motor vehicles. Driveway Edge and Border Treatments A driveway edge should be clearly defined and visible to all users, so users can ascertain the lat- eral limits of motor vehicle operation. From observations such as in Exhibit 5-46, a vertical wall at the edge of a driveway causes drivers to shift their vehicles toward the center line. It is suggested that no vertical face (e.g., a retaining wall) be within 2 feet of the edge of the intended way for vehi- cle use. A wider offset must be provided if there will be a sidewalk parallel to the driveway. For driveways with flat edges (i.e., no curb) in a fill, drivers will find it harder to determine where the edge is in rain, fog, or darkness. The designer should not place a sudden drop off at such an edge. A relatively flat shoulder with a minimum width of 2 ft (after any rounding) is suggested before the side slopes downward. Some property owners install reflectors or other similar devices at the edge to help deal with this problem. As shown in Exhibit 5-47a, the toe of a slope should not extend to the base of a driveway or sidewalk edge, because runoff and erosion can lead to a mud-covered driveway or sidewalk surface. Exhibit 5-47b shows the toe of the slope recessed from the pavement edge, a method which yields better results. Edge Clearance from Fixed Objects Fixed objects such as utility poles, fire hydrants, and drainage inlets should be set back from the edge of the driveway and from the edge of the roadway. Reasons for this include allowing Exhibit 5-43. Suggested minimum lengths of straight driveway alignment. Driveway Category Description of Common Applications* Minimum Length of Straight Alignment on the Driveway Approach Adjacent to the Connection Transition with a Public Roadway STANDARD DRIVEWAYS Very high intensity Urban activity center with almost constant driveway use during hours of operation. 75 ft (based on length of 3 P-car) Local requirements for tangent at a signalized intersection may apply. Higher intensity Medium-size office or retail (e.g., community shopping center) with frequent driveway use during hours of operation. 50 ft (based on length of 2 P-car) Medium intensity Smaller office or retail, some apartment complexes, with occasional driveway use during hours of operation. 25 ft (based on length of 1 P-car) Lower intensity Single-family or duplex residential, other types with very low use. May not apply to rural residential. 25 ft (based on length of 1 P-car) SPECIAL SITUATION DRIVEWAYS CBD, Farm or ranch, Field, Industrial A mix of design vehicles. Length equal to the design vehicle length, plus 5 ft NOTE: The recommended lengths are based on orienting a likely number of vehicles to be present up to and through the driveway-roadway intersection. Further study to develop these values is needed. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category. 52 Guide for the Geometric Design of Driveways Exhibit 5-44. Lack of a sidewalk forces the pedestrian into the driveway.

Geometric Design Elements 53 Exhibit 5-46. Effects of a vertical wall too close to the driveway. retaining wall is too close to driveway traffic; to have a comfortable distance away from the wall, the driver moves over, across the centerline Exhibit 5-45. Suggestions for separate facilities for bicyclists and pedestrians. Driveway Category Description of Common Applications* Need for a Facility Parallel to Driveway for Bicyclists or Pedestrians STANDARD DRIVEWAYS Very high intensity Urban activity center with almost constant driveway use during hours of operation. Bicycle – the need for separate lane or path depends on bicycle use in the area Pedestrian – often need sidewalk Higher intensity Medium-size office or retail (e.g., community shopping center) with frequent driveway use during hours of operation. Bicycle – shared use may be adequate Pedestrian – may need sidewalk Medium intensity Smaller office or retail and some apartment complexes with occasional driveway use during hours of operation. Bicycle – shared use usually adequate Pedestrian – may need sidewalk Lower intensity Single-family or duplex residential, other types with low use. May not apply to rural residential. Shared use is adequate SPECIAL SITUATION DRIVEWAYS CBD Seldom applicable, because buildings are close to the street Building faces are close to the street. Farm or ranch, Field Shared use is adequate Seldom used, very low volume. Industrial Depends on the specific site plan and transportation modes used by the employees. Driveways are often used by large vehicles. May have separate driveways for employees and/or customers. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.

54 Guide for the Geometric Design of Driveways clearance for vehicle side mirrors and to account for the wheel and body paths of offtracking turning vehicles. Exhibit 5-48 shows drainage inlets flanking a driveway, with both inlets show- ing what appears to be damage from turning vehicles. Example design criteria such as Exhibit 5-49 suggest a clearance from vertically projecting fixed objects (e.g., poles and fire hydrants) to the edge of the driveway of 5 feet or more. Objects such as curb inlets should clear the paths of vehicles turning into and out of the driveway. Clear zone design practices affect the lateral placement of objects with respect to the edge of the traveled way of a street or highway. The minimal urban clear zone may be inadequate in the immediate vicin- ity of the roadway-driveway intersection, and a larger dimension may better accommodate turn- ing vehicle offtracking. The adequacy of any given driveway design can be checked with the turning templates of the design vehicle. The designer should also check to ensure that roadside objects do not become obstacles for other users (e.g., bicyclists and pedestrians). Exhibit 5-47. Locating the toe of a slope near a driveway. if the toe of the slope abuts the edges of the driveway or the sidewalk, then mud running down the slope can accumulate on the driveway and sidewalk, leaving a messy area (a) Toe of Slope moving the toe of the slope back from the edges of the driveway and the sidewalk leaves space for run off to accumulate, making it less likely that mud will cover the driveway or sidewalk roadwayroadway dri vew ay dri vew ay (b) Toe of Slope Exhibit 5-48. Drainage inlets too close to the edges of the driveway. Exhibit 5-49. Driveway edge clearance from fixed objects.

Geometric Design Elements 55 Driveway Length The following sections address the design of the following elements related to driveway length: 1. Minimum length of driveway to a barrier (e.g., garage door or gate) 2. Minimum length of driveway paving 3. Minimum length of the driveway throat or connection depth 4. Accommodating the need to reverse vehicle direction (i.e., turn around) within the private site. Drawings showing different facets of driveway length were provided at the beginning of Chapter 2 as well as on the following pages. Driveways can be divided into two groups. One group operates in the manner typical of single- family suburban driveways, at which vehicles enter the driveway and then come to a stop and park. The other group operates in a manner typical of commercial driveways—entering vehicles continue to move and proceed for some distance along the driveway, often into a parking lot. Some of the following controls are more likely to apply to the first category, while other controls are more likely to apply to the second category. Minimum Length of Driveway For this section, the length of a driveway is the distance from where the driveway on one end connects to the traveled way, to the other end where the driveway encounters some sort of barrier or terminates. This may be an intersecting circulation road within a site, the end of the pavement, a gate, a garage door, or other barrier that when in place, discourages or pre- vents a motor vehicle from proceeding. Even under the simplest of situations, many driveways will require a certain minimum length in order to avoid creating problems for one or more user groups. Driveway Minimum Length Considerations Problems can result when vehicles entering a driveway cannot proceed far enough into the driveway and parts of the vehicle then block the traveled way, bicycle lanes or paths, or sidewalks. Exhibit 5-50 shows vehicles parked in the driveways of townhouses constructed in the early 2000s. The rears of these vehicles partially block the sidewalk and lumber in the bed of the pickup truck in the foreground extends over the tailgate into the pedestrian path. This situation would be especially dangerous for a pedestrian with visual impairment using the sidewalk. Unless a driveway is so short as to discourage its use for stopping or parking, the minimum length between controlling features on each end of the driveway is the sum of the following three components (see Exhibit 5-51): 1. Setback from the end toward the roadway to clear the outer edge of the traveled way, a bicycle lane or path, or a sidewalk 2. Length of the longest vehicle that typically would park there 3. Clearance buffer from a gate, garage door, or other similar end-barrier The buffer allows a person to walk between the end of the vehicle and the end-barrier. The buffer should also accommodate many drivers’ tendencies to shy away from a barrier, rather than pulling close to it. It is hoped that the driver of a vehicle with a load that slightly hangs over the rear will use the buffer to pull forward until the load clears the sidewalk.

56 Guide for the Geometric Design of Driveways Exhibit 5-50. Insufficient driveway length leads to vehicles blocking the sidewalk. Exhibit 5-51. Minimum driveway length considerations. ++ driveway sidewalkroadway Where stopping or parking in driveway occurs, Minimum Driveway Length = sum of 1. setback to outer edge of sidewalk, or other similar control 2. design vehicle length 3. buffer Driveway Minimum Length Design Suggestions Given that the minimum length of the driveway is the sum of three values, two of which can vary greatly, prescribing a single or even a few values is of little benefit. Instead, it is recommended that the designer follow this series of steps: 1. Determine the longest vehicle type likely to use the driveway. 2. Determine the length of that vehicle. 3. Estimate a front buffer dimension. In the case of a smaller design vehicle (e.g., a P-car), esti- mate 2 feet. For a larger design vehicle (e.g., a bus or large truck), select 3 feet. If the front buffer area involves a gate that swings outward, there also should be an allowance for the gate. 4. Estimate a value for the rear clearance. Where a sidewalk exists, this is the distance from the edge of the traveled way to the far edge of the sidewalk. If no sidewalk exists, allow a mini- mum of 2 feet. 5. Sum these values to determine the minimum driveway length. Research would be helpful to better define these dimensions (e.g., the actual buffer taken by the drivers of various sizes of vehicles). Minimum Length of Driveway Paving If the driveway within the private property site is dirt or gravel, how far back from the edge of the traveled way to pave the driveway connection is an issue. A survey of transportation agencies found that practices varied among agencies, and no one practice predominated. Some agencies pave the driveway connection a fixed distance from the edge of the traveled way; others pave to the right-of-way line. The objectives of paving the connection to a gravel or dirt driveway some distance back from the traveled way edge include (1) providing a more stable driveway surface “platform” from which to enter or exit the traveled way and (2) minimizing or eliminating the depositing of dirt, gravel, or mud onto the traveled way. Factors which can affect the extent to which debris from such a private driveway are deposited on the traveled way include • The distance from the traveled way edge to the beginning of the gravel or dirt surface; • The grade of the driveway; • Surface drainage patterns, combined with the amount of precipitation; and • The volumes and types of traffic using the driveway.

Given the lack of consensus among agencies and the factors that affect the extent to which debris from unpaved driveways is deposited on the traveled way, guidance on this matter is limited to advising designers to pave driveway connections some distance back from the edge of the traveled way. A designer may find guidance by observing the extent of the debris ema- nating from existing unpaved driveways in the vicinity of the driveway under consideration. One specific suggestion is to pave a length at least as long as the length of the vehicles expected to use the driveway, plus a clearance from the edge of the traveled way or sidewalk. This will encourage vehicles that pull off the road to clear the roadway and sidewalk. Another suggestion is that local governments require on-site paving or other mitigation actions to prevent debris from washing onto the public roadway. Minimum Length of Driveway Throat The driveway throat length is the distance from the outer edge of the traveled way of the inter- secting roadway to the first point along the driveway at which there are conflicting vehicular traf- fic movements. Similar or related terms include the driveway connection depth, reservoir length, stacking distance, and storage length. Exhibit 5-52 illustrates a driveway throat. Sources differ as to what point actually defines the internal (i.e., within the private site) end of the throat. Examples of these variations include “end of the driveway inside the land devel- opment” (5-17), “the parking lot served by a driveway” (5-18), and “the furthest end of the driveway” (5-19). Given that an impetus for providing an adequate length for the driveway throat is related to allowing smooth traffic flow along the driveway between the street and on- site roadways or parking lots, the point at which conflicting traffic movements are encountered was selected to define the end of the driveway throat that lies within the site. So, implicit in the definition used herein is “non-conflicted” throat length. Design Considerations Providing an adequate driveway throat length or connection depth in which there are no con- flicting movements can help create smoother traffic flows in and near the driveway throat and avoid conflicts to which drivers may not have adequate time to react (which in turn may lead to colli- sions). As Exhibit 5-52 shows, an inadequate throat length can produce traffic situations that adversely affect the flow of traffic on the public roadway. In this drawing, vehicles that have entered the driveway have formed a queue that blocks the sidewalk. Additional vehicles trying to enter the driveway will not be able to proceed; therefore, they will stop in and block the public roadway too. Exhibit 5-53 portrays a different situation that can affect the length of the right or entry side of the throat. All parking spaces should be far enough from the roadway, bike path, and/or sidewalk, so that vehicles backing out of parking stalls do not encroach into the projection of the sidewalk or bike path across the driveway, or into the roadway. Even if a vehicle backing out of a parking space into the driveway throat does not encroach into one of these areas, the backing vehicle will Geometric Design Elements 57 Exhibit 5-52. Driveway throat. parking lot THROAT LENGTH or CONNECTION DEPTH ro a dw ay driveway in te rn al (s ite ) r oa d sidewalk Exhibit 5-53. Driveway entry throat and parking stalls. ro a dw ay VEHICLE BACKING OUT OF PARKING STALL sidewalk

58 Guide for the Geometric Design of Driveways still block the driveway entry for the duration of the backing maneuver. In many situations, such a blockage would have undesirable effects on traffic, so in those cases, parking should not be allowed in the driveway throat area. Exhibit 5-54 shows yet another type of conflict, a speed hump installed in a driveway entry. Observations of traffic at this driveway revealed that as vehicles turned into the driveway, drivers were surprised by the need to rapidly decelerate over a short distance. Such a rapid deceleration increases the driver’s exposure to being struck on the side by through vehicles and in the rear by following vehicles. The process of slowing or stopping before turning into this driveway constitutes adequate traffic calming. When turning into a driveway while watching for conflicting bicyclists, pedestrians, and other vehicles, motorists should not be confronted with additional driving tasks until they have had time and distance to reorient themselves. The following factors affect the distance needed to provide an adequate driveway throat length: • The positions of bike paths and sidewalks. • The queuing or stacking space needed for exiting vehicles. If the exit is signalized, then suffi- cient queuing length is needed to supply the green phase with vehicles proceeding at the sat- uration flow rate, accounting for lost time due to weaving on the driveway approach during the green phase. • In cases of a multilane exit, the length needed for exiting vehicles to make weaving maneuvers as they change lanes in the driveway. • The distance needed to provide motorists entering the driveway with time to reorient them- selves and detect conflicting traffic movements from crossroads, parking spaces, bicycle routes, or pedestrian paths they encounter. • The queuing or stacking space needed for entering vehicles. • For a multilane entry, the length for entering vehicles to make weaving maneuvers as they change lanes in the driveway. • For a one-way driveway, enough length to place Wrong Way/Do Not Enter signs so that the intent is obvious to motorists. Other considerations for the length of the throat found in the literature review include the • Functional category of the intersecting roadway, • Type of driveway intersection traffic control (stop sign or signal) and traffic signal timing, • Type and intensity of land use activities served, and • Number of parking spaces (within the site) per exit lane. Exhibit 5-54. Undesirable speed control in driveway throat.

Given that one of the underlying factors to consider is the driveway volume, both entering and exiting vehicles during the peak time period, it may be necessary to estimate likely driveway usage before designing the length of the driveway throat. Design Suggestions The throat length must be long enough to avoid internal site conflicts associated with cross- ing or weaving movements. It also must be adequate to avoid spillback onto the public road or internal circulation system. There are different controls: 1. Designing sufficient length to react to conflicts, 2. Designing sufficient length to accommodate traffic queues, and 3. Designing sufficient length to accommodate weaving. Different sources have developed different approaches for establishing minimum throat lengths. The following narrative presents the approaches from various sources. Koepke and Levinson Throat Length. When more detailed, site-specific information is available, one could apply the recommendations by Koepke and Levinson in NCHRP Report 348 (5-20). For signalized driveways, suggested on-site throat lengths (per lane) were based on the equation N = 2qr, where N = number of cars to store, q = vehicles per hour per lane, and r = effective red time per cycle. Alternative guidelines were cited based on the number of parking spaces per exit lane for multi-family, residential, retail, office, and industrial uses. The following suggested guidelines were based on both sets of criteria: • 50 feet for minor driveways that serve 50 to 100 apartments, less than 50,000 square feet of retail, or a quality restaurant; • 150 feet, with at least two exit lanes, for shopping centers of up to 700,000 square feet, and office complexes up to 500,000 square feet; and • 200 feet or more, with at least two exit lanes, for larger commercial complexes. Stover and Keopke Throat Length. In Transportation and Land Development, Stover and Koepke (5-13, p. 7–28) state that the exit condition controls the throat length for high-volume traffic generators, while the entry condition controls the throat length for low-volume traffic generators. The exit side of a driveway should be designed to enable traffic to efficiently leave a site. The throat length and cross section are interrelated: the wider the cross section, the longer the exit throat length needed to accommodate the associated weaving maneuvers. Exhibit 5-55 presents the minimum throat length for stop-controlled and for signalized-access drives, based on the number of egress or exit lanes. Geometric Design Elements 59 Exhibit 5-55. Minimum throat length based on the type of control and number of lanes. Type of Number of Exit Lanes Present Control 1 Exit Lane 2 Exit Lanes 3 Exit Lanes 4 Exit Lanes STOP sign 30 to 50 ft 50 ft (2 cars) -- -- Signal -- 75 ft 200 ft 300 ft NOTE: -- indicates no value given Sources: Transportation and Land Development, 2nd ed. (2002), p. 7-28 (5-13) and Access Management Manual (2003), p. 184-185 (5-21)

60 Guide for the Geometric Design of Driveways The throat length for a one-way exit driveway needs to be sufficient to allow for DO NOT ENTER or WRONG WAY signs to be installed to be effective in warning drivers turning from the roadway. This is related to liability, not queue length. The throat length based on the entrance side of a driveway needs to minimize the potential for the conflicts on the access drive from adversely affecting the intersecting roadway. Drivers enter- ing the site should clear the roadway intersection before encountering decision points and poten- tial conflicts along the driveway. Transportation and Land Development indicates the minimum throat lengths for unsignalized access drives based on two driveway configurations – 1 entering/ 1 exiting lane and 1 entering/2 exiting lanes. For the 1 entering/1 exiting lane configuration, the minimum throat length is 75 feet to the first parking spaces on site or 30 feet to the first intersection on site. For the 1 entering/2 exiting lane configuration, the minimum throat length is 75 feet to the first parking spaces on site or 50 feet to the first intersection on site. For high- volume traffic generators, it is the exit condition that governs the needed throat length. Roseville Throat Length. The Roseville, California, design standards (5-22) present a detailed procedure for estimating the needed length of the driveway throat. Agencies may apply the throat length criteria from other sources to help establish a similar type of procedure. The Roseville procedure is part of the traffic impact study process that applies to proposed proj- ects estimated to generate more than 50 PM peak-hour trip ends. The traffic study includes an eval- uation of the Minimum Required Throat Depth (MRTD) needed on-site for each access point for a proposed development. The MRTD requirement does not apply to single-family residential or duplex uses. The MRTD is measured from the back of the sidewalk to the first drive aisle or park- ing stall. The purpose of the MRTD is to allow enough stacking distance for egressing vehicles so that the first drive aisle or parking stall is not blocked. This minimizes the possibility of incoming vehicles queuing out into the traveled way of the main street thereby creating a safety concern. The MRTD is measured in car length increments of 25 feet and rounded up to the nearest multiple of 25 feet. The City does not allow a MRTD of less than 25 feet for any project. Throat depths greater than the calculated MRTD are encouraged. On-site parking is not permitted within the MRTD area. The MRTD is a function of the length of the queue of vehicles waiting to exit the driveway. The length of this queue is a function of two variables: the number of vehicles desiring to egress dur- ing a given time period versus the number of vehicles that can enter the traffic stream of the main road during that same time period. If the calculated MRTD is physically or unreasonably too long for the proposed development, then the traffic study can suggest ways to reduce the MRTD by either decreasing the egress demand volume, or by increasing the movement capacity. There are cases when an MRTD of 25 feet is acceptable, for example, when the first drive aisle is “one-way only” as shown in Exhibit 5-56. Another scenario where a MRTD of 25 feet is accept- able is when a raised center median is constructed in the driveway throat from the back of the sidewalk to the calculated MRTD distance. In this case, the nearest drive aisle can be two-way, but turning movements into and out of the drive aisle are restricted by the raised median. Because of the different operations at signalized and unsignalized driveways, two different methodologies apply. At unsignalized project driveways, the MRTD is based on a series of regression equations that the City uses to estimate maximum queue lengths at minor stop- controlled intersections. These equations apply the methodology presented in “Estimation of Maximum Queue Lengths at Unsignalized Intersections” from the November 2001 ITE Journal. Exhibits 5-57 and 5-58 present the methodologies used for calculating the MRTD for various unsignalized driveway conditions. Major street volumes are based on projected future traffic volumes from the latest version of the citywide traffic model. Alternative methodologies for cal-

Geometric Design Elements 61 Exhibit 5-56. Roseville explanatory drawing. Exhibit 5-57. Minimum required throat depth regression equations. culating unsignalized MRTD lengths may be considered, but need to first be approved by the Public Works Department prior to incorporation into traffic studies. At signalized project driveways, MRTD lengths are a function of egressing traffic volumes, lane geometrics, and traffic signal timing. Typically, signalized access locations will have more than one approach lane for egressing vehicles; therefore, the MRTD is determined from the lane with the longest queue. The MRTD is based on the Operational Analysis methodology contained in

62 Guide for the Geometric Design of Driveways the latest version of the Highway Capacity Manual or other methodology as approved by the City’s Public Works Department. Major street volumes are based on projected future traffic volumes from the latest version of the citywide traffic model. For existing traffic signals, it is recommended that the consultant discuss likely signal timing parameters with City staff. There may be some restrictions to signal timing parameters for existing signals because of progression and so forth. The City also has provisions to help ensure that sufficient onsite storage is provided for drive- through service uses to ensure that vehicles will not queue into the public right-of-way. The following definitions are for the terms used in the MRTD equations: AppVol = hourly traffic volume divided by peak-hour factor (PHF) for subject movement ConflVol = hourly traffic volume divided by PHF that conflicts with subject movement (refer to the Highway Capacity Manual to identify movements that conflict with subject approach) TS = a dummy variable with a value of 1 if a traffic signal is located on the major street within one-quarter mile of the subject intersection and 0 otherwise Lanes = number of through lanes occupied by conflicting traffic Speed = posted speed limit on major street (in miles per hour) RT% = Percentage of vehicles on shared left/through/right minor street approach that turn right The following scenario employs several assumptions to illustrate another facet of principles related to adequate throat length—minimizing traffic conflicts of the entry side of the driveway throat. Assuming a level, 90-degree entry, it was hypothesized that as drivers turn right into a driveway, the eventual 90-degree reorientation of drivers’ lines of sight is affected by factors such as the following: Exhibit 5-58. MRTD for right-turn-only movements.

1. Human factors. The span or width of drivers’ fields of vision and the degree to which drivers can turn their heads. 2. Vehicle factors. Limitations imposed by the structure of the vehicle (e.g., the position and width of the front windshield posts). 3. Operational factors. Informal observations suggest that drivers maneuvering vehicles into a driveway are not free to devote attention to conflicts in the driveway throat length ahead until after the entering vehicle has cleared any conflicts at the entry. This includes conflicts with pedestrians and “sideswipe conflicts” between the left front corner of the entering vehicle and the left side of any vehicles exiting the driveway onto the public street. Assume that drivers turning right into a driveway with a 25-ft radius at a speed of 15 mph or 22.0 ft/s give full attention to the driveway ahead after completing 60 degrees of the turn. At this point, drivers have sufficiently squared-up their lines of sight and can detect a conflict (e.g., a vehicle backing out of a parking stall or cross traffic within the site). The current AASHTO Green Book guidelines (5-1, pp. 110–114) allow 2.5 seconds for a driver to react to an unexpected sit- uation ahead requiring the vehicle to stop; whether or not a conflict in the driveway ahead would constitute an unexpected situation is arguable. For this illustration, assume that the driver has a narrow focus on the driveway ahead and requires only 1.0 second of perception-reaction time and a deceleration rate of 11.2 ft/s2. This leads to the following calculations: 25 ft radius × tan (90° − 60°) = 14.4 ft 25 ft tangent − 14.4 ft = 10.6 ft into driveway from edge of traveled way Distance from driver to front bumper of vehicle: 6 ft Distance into driveway + perception reaction distance + braking distance 10.6 ft + 6 + [22.0  1.0] + [0.5 × 22.02 / 11.2] = 60 ft of entry throat length Therefore, the designer would require a minimum of 60 feet of driveway connection depth from the outer edge of the traveled way to the first crossroad or other conflicting movements within the site. If the first conflict encountered is with a vehicle backing out of a parking stall, then the position of the rear bumper of the vehicle that has just backed out of the stall will also need to be taken into account. Even with this non-conflicting connection depth, if a second vehicle is closely following the first and also turning right into the driveway, the driver of the second vehicle may not be able to react and stop or the second vehicle may come to a stop with its rear still in the through roadway. Other Throat Lengths. For a comparison, Exhibit 5-59 presents minimum throat length criteria from two states, New Mexico and Florida. In both cases, the minimum requirement is 30 feet. Geometric Design Elements 63 Exhibit 5-59. Throat length criteria from two states. Source: New Mexico DOT, State Acc. Mgmt. Manual Ch. 8, Sec. 18, p. 91, Sept. 2001 FDOT Driveway Handbook, p. 54, Mar. 2005 Source: Vergil Stover unpublished course notes (b)(a)

Providing Onsite Turnaround Capability It is often undesirable or unsafe for vehicles to perform a backing maneuver from a driveway into a public roadway. Therefore, most sites should be designed so that once entering the site, a vehicle can be re-oriented and leave in a forward direction. This is highly desirable for all sites except for single-family and duplex residences along lower volume, lower speed streets. The type and design of needed turnaround facilities depend on the size of and types of activ- ities conducted on the site, the likely mix of vehicles, the building arrangements, and the circu- lation system for each site. Sometimes, turnaround needs can be accommodated by circulation on internal road systems or through parking areas. In other situations, a site needs a specific turn- around facility, such as a circle or other shape shown in Exhibit 5-60. Internal Roadway Systems Many developments, especially larger ones, have internal circulation systems that allow vehi- cles to enter the site and then, through a series of normal driving maneuvers, assume an orienta- tion that allows the vehicle to head out of the site. Circulation Through Parking Lots Somewhat similar to an internal roadway system but on a smaller scale, other sites have a layout that allows vehicles to circulate through the parking lot and leave the site in a forward direction. This type of turnaround works well with cars and smaller trucks, but may not be adequate for large trucks, unless greater maneuvering spaces are provided. One form of this is “Loop Routing,” shown in Exhibit 5-60 (a and b). Where two parallel drive- ways enter a site, it is sometimes practical to turn around via an inverted “U” movement. For traf- fic driving on the right side, a counter clockwise movement has less internal traffic conflict, and left turns around corners are easier for larger vehicles to negotiate than are right turns. Sometimes, a single driveway access point can be “split” via a loop road to provide the turnaround. This pat- tern is often seen at fast food restaurants, with the building located inside the loop. 64 Guide for the Geometric Design of Driveways Exhibit 5-60. Turnaround design schematics. T-shape Hammerhead roadway roadway U-shaped internal circulation pattern U-shaped internal circulation pattern Offset cul-de-sac Cul-de-sac should have a reverse curve (a) (b) (c) (d) (e) (f)

Geometric Design Elements 65 Specific Turnaround Facilities When a site does not include features such as an internal roadway system or parking lot circu- lation that allow a driver to re-orient a vehicle and proceed from the site in a forward direction onto the public roadway, then specific turnaround facilities may be needed. One common form is the cul-de-sac or circular turnaround. A circular turnaround can be cen- tered on the driveway or offset to one side. Circular turnarounds are generally preferable, although T-shaped and Y-shaped (hammerhead) turnarounds may be used. Exhibit 5-61 offers minimum desirable circular turnaround dimensions (5-23). With the T- and Y-shapes, vehicles turn left into the special roadway at the end of the driveway, back across the drive, and then proceed forward to turn left into the driveway. For passenger cars, a 60- by 20-ft area is needed. Advantages of T- or Y-shaped turnarounds are that they have lower construction and maintenance costs and require less land than circular turnarounds. Because T- or Y-shaped turnarounds require all vehicles to make a back-up movement, their application is limited to very low-volume driveways. Driveway Vertical Alignment Elements This section provides guidelines for designing the vertical alignment (or profile), which con- sists of grades and vertical curves. Designers should establish a vertical alignment that allows vehicles to conveniently and expeditiously enter and exit the driveway. Designers should avoid profiles that allow the underside of a vehicle to drag or hang-up. When establishing the vertical alignment of the driveway, the designer must consider limitations on the sidewalk cross slope to accommodate pedestrians and pedestrians with disabilities. Also, designers should check the profile to make sure it is not creating drainage problems. Vertical Clearance For many driveways, vertical clearance is not an issue. But where there are overhead struc- tures, utility lines, or vegetation, the designer should check that the vertical clearance is adequate for the design vehicles. Exhibit 5-61. Minimum desirable circular turnaround dimensions. Vehicles Accommodated Radius to Face of Outer Curb (in feet) Comments Passenger cars only 30 Passenger cars, school buses, delivery trucks, emergency vehicles 42 A 45-ft radius would allow a central landscaped island with an inside curb radius of 25 feet and a 20-ft- wide turning roadway. However, if a passenger vehicle is parked on the turning roadway, this geometry may not accommodate ambulance, fire truck, or solid waste vehicles. Curb parking is permitted 50 Where a landing is provided at the base of the circle, a tangent section of 22 feet for each car should be provided. Source: Kulash, Residential Streets, 3rd ed., Institute of Transportation Engineers, Washington, DC © 2001. Used by permission

66 Guide for the Geometric Design of Driveways Sidewalk Cross Slope (Driveway Grade) When this guide was prepared, the ADA design requirements for accessibility related to pedes- trian facilities in the public right-of-way were still being developed. Although only draft accessi- bility guidelines and other technical assistance advisory documents from federal sources were available, the ADA does and will continue to apply to sidewalks, curb ramps, and pedestrian crossings at driveways that are newly constructed or altered since January 26, 1992. The scop- ing provisions of these draft guidelines define where and to what degree accessibility within the public right-of-way is required and state the following: R201.1 Scope. All newly designed and newly constructed facilities located in the public right-of-way shall comply with these requirements. All altered portions of existing facilities located in the public right-of-way shall comply with these requirements to the maximum extent feasible. The ADA defines “facilities” very broadly and the US Department of Justice states in its ADA Title III regulations that this definition “ . . . includes both indoor and outdoor areas where human-constructed improvements, structures, equipment, or property have been added to the natural environment” (see 56 Fed. Register page 35550). The pedestrian crossing at a driveway is a facility covered by the ADA and thus must be made accessible in new construction projects and must be made accessible to the maximum extent feasible in alteration projects. The draft ADA guidelines for public rights-of-way basically require that there be a continuous accessible pedestrian route (i.e., PAR) leading up to and crossing each driveway. This will typically include the following: • The transition between the public sidewalk and the pedestrian crossing (marked or unmarked) at the driveway, which is usually achieved by means of an accessible curb ramp; • The pedestrian cross walk pavement surface; and • Any island improvements within the driveway that pedestrians must traverse. The pedestrian crossing at newly constructed driveways must offer a minimum 48-in.-wide route with a cross slope no greater than 2 percent. Where the driveway is an alteration to existing improve- ments within the public right-of-way, the pedestrian crossing portion must offer a cross slope no steeper than 2 percent to the maximum extent feasible, given existing site-related constraints. Site-related constraints that may prohibit strict compliance with the ADA maximum 2 percent cross slope specifications include severely limited right-of-way or sidewalk width in which to nego- tiate the vertical rise between the roadway elevation and the parking area, or steep existing grades on an adjoining, densely developed property that the driveway serves. Engineering judgment plays a key role during the design of driveway alteration projects where full accessibility is not being offered and that judgment may be challenged under ADA by experts analyzing every detail of the design and site factors that may or may not be found to justify any alleged access barriers created by the design. Exhibit 5-62 shows a driveway grade rising quickly from the gutter line, creating an excessive and unacceptable cross slope for the pedestrian path. The five illustrations constituting Exhibit 5-63 (taken from the recommendations of the US Access Board’s Public Rights-of-Way Accessibility Advisory Committee published in “Building a True Community”) demonstrate driveway design options that comply with the accessibility specifications in the draft ADA guidelines. • Option A, Ramp Sidewalk, shows the sidewalk simply ramping down at each side of the drive- way with a maximum 2 percent or 1:48 cross slope along the pedestrian crossing. • Option B, Apron Offset Sidewalk, shows a directional offset in the sidewalk to avoid the steep cross slope that would otherwise be created by crossing the driveway apron on the steeply slop- ing portion.

• Option C, Gutter Bridgeplate, shows the whole width of the sidewalk having a limited cross slope and employs a bridgeplate over the gutter at a rolled curb condition to limit the likeli- hood of vehicles bottoming out. • Option D, Wide Sidewalk, uses the rear most 48 inches of the driveway apron to cross the drive without a cross slope steeper than 2 percent. • Option E, Setback Sidewalk, shows how a more traditional returned curb style driveway apron can be installed between the gutter and the street side of the sidewalk which adjoins a land- scaped green space. At pedestrian crossings in driveways of more developed commercial sites where the drive- way more closely resembles a street, the design and construction of the pedestrian crossing area between the curb ramps is subject to the same 2% maximum cross slope that other drive- ways are subject to. In Exhibit 5-64, the driveway leading into a regional shopping mall looks similar to a roadway intersection; the curb ramps and accessible pedestrian crossings should be constructed with a maximum 2% cross slope. Geometric Design Elements 67 Exhibit 5-62. Example of driveway grade creating unacceptable sidewalk cross slope. Exhibit 5-63. Examples of driveways that comply with accessibility specifications. Exhibit 5-64. Driveway entrance at regional mall.

68 Guide for the Geometric Design of Driveways Driveway-sidewalk crossing transitions call for special attention. Of particular concern are the multi-dimensional tapers that arise from dust-pan and similar flared treatments. The 2004 AASHTO pedestrian design guide points out that “side flares and cross slopes at driveway aprons may cause a drive wheel, caster, or leg tip to lose contact with the surface” (5-2, p. 61–62). Therefore, such flares should not be used unless there is another suitable PAR, such as might be provided by a wide sidewalk. Driveway Grade (Sidewalk Cross Slope), Change of Grade, and Vertical Alignment Three types of control for the design of the driveway profile are physical, operational, and drainage: • Physical controls call for a design that maintains enough clearance so the underside of a vehi- cle does not drag on the roadway or driveway surface. This control is necessary for all drive- ways, even one connecting to an alley. Because of the changes in vertical profile grade often found at driveway entrances, these locations are among the more vulnerable to hang ups when the undercarriage of the vehicle comes into contact with or drags the pavement surface. • Operational controls dictate a vertical alignment for the driveway that allows a convenient and safe entry with minimal conflicts. To achieve this, the changes of gradient must not be too abrupt. This is especially important on driveways that intersect higher volume or higher speed roadways. Operational problems may arise from certain combinations of vertical profiles and vehicles. One problem is vehicle-occupant discomfort due to poor vertical alignment such as bumps, steep grades, and abrupt changes in grade. In extreme cases, there may be restricted sight distance, which affect safety adversely. In addition, excessive differences in speed between through vehicles and vehicles turning into or out of the driveway, because of the vertical pro- file, can also increase vehicles’ exposure to crashes. • Drainage, requires a profile that does not create undesirable drainage patterns. It may be unac- ceptable for surface runoff in the gutter to flow into the driveway opening and onto private property. Physical Vehicle Ground Clearance Control As Exhibit 5-65 shows, the underside of a vehicle entering or exiting a driveway can drag on either a crest or a sag alignment with an abrupt change of grade. Any excessive grade change between the cross slope of the roadway and the driveway grade, between the driveway grade and an intersecting sidewalk, or between successive driveway grades can cause a vehicle to drag (see Exhibit 5-66). Vehicles with low ground clearance and a long wheelbase or overhang can even become lodged (also referred to as “hung up” or “high-centered”) on alignments with sharp grade changes. At best, hang-ups result in some vehicular delay and minor damage to the undercarriage of the vehicle and to the pavement surface. At worst, a crash can occur. Exhibit 5-65. Geometry of ground clearance dragging. WB=wheelbase (b)(a) OHF= front overhang OHR= rear overhang WB=wheelbase OHF= front overhang OHR= rear overhang roadway curb roadway OHR SAG: Have problem if axle-to-bumper underclearance is inadequate. For symmetrical sag, is critical when one axle is at distance WB from the low point. CREST: Have problem if axle-to-axle underclearance is inadequate. For symmetrical crest, is critical when axles are a distance WB/2 from the high point. WB/2 driveway WB/2 WB curb OHF driveway

To design the vertical alignment elements, the designer needs to determine an appropriate design vehicle. As previously discussed, several types of long-wheelbase, low-ground-clearance vehicles can be expected to use some driveways, including articulated beverage trucks, car carri- ers, and passenger car-trailer combinations. The design vehicle for vertical alignment may be dif- ferent from the design vehicle used to design the horizontal alignment (e.g., turning radii). The designer also needs to have a general understanding of the shape of the vertical profile to be nego- tiated by the design vehicle. This includes, for example, the roadway cross slope, the driveway grade line, and other controls (e.g., locations and elevations of intersecting sidewalks). Using reasonable care in selecting a design vehicle and designing the vertical elements to accommodate that vehicle will not completely preclude hang-ups, dragging, or other operational problems from occurring. A vehicle longer and/or lower than the design vehicle may enter a driveway and encounter problems. To meet the needs of shippers, commercial vehicle manufac- turers continue to introduce longer and/or lower vehicles and new vehicle configurations that will require periodic updating of the list of design vehicles. Similarly, as property changes hands or as redevelopment occurs, the nature of the land use served by the driveway may change over time. A different class of vehicle than originally intended may use the driveway. Although this is beyond the control of the designer, it offers an explanation of why hang-ups may happen at locations where they formerly did not occur and represents an issue to be addressed in the permitting process. Also, a vehicle for which the vertical elements have been appropriately designed may encounter problems at a particular driveway. This could be due to vehicle loading condition (e.g., an over- loaded vehicle) that reduces actual ground clearance to something below the design value. The vertical profile is subject to changes over time. For example, the roadway may be milled or resur- faced such that its elevations and cross slopes change. In addition, the roadway and/or driveway (and associated features such as sidewalks) may deform over time due to applied loads, the effects of weather, or construction deficiencies. As mentioned above, the vertical profile(s) used in design should be that reasonably expected to be used by the design vehicle. The possibility always exists that a design vehicle will follow an unusual or out-of-the-ordinary path in negotiating the drive- way such that hang-up or dragging could result. Exhibit 5-67 shows maximum uphill and downhill grades, as reported by transportation agencies in a survey. Geometric Design Elements 69 Exhibit 5-66. Driveway with multiple scrapes from underside dragging.

70 Guide for the Geometric Design of Driveways Maximum allowable grade, by itself, is not a sufficient control. What matters is the difference between successive grades, or the change of grade. The change of grade is what creates the crests and sags that cause the underside of a vehicle to drag. Although perhaps not widely recognized, guidance on vertical geometry applicable to driveways has been available for some time. The sec- tion on railroad-highway grade crossing design in AASHTO’s policy on geometric design (5-1) provides recommendations on designing the vertical profile at grade crossings. AASHTO rec- ommends that the crossing surface be in the same plane as the top of rails for a distance of 2 feet outside of the rails, and that the surface of the roadway be not more than 3 inches higher or lower than the top of the nearest rail at a point 30 feet from the rail, unless track superelevation dic- tates otherwise. Similarly, a 1987 ITE guideline for driveway design discussed vertical alignment. Eck and Kang (5-24) used a vehicle with a 36-ft wheelbase and 5 inches of ground clearance to analyze a maxi- mum grade change of 3 percent (for low-volume driveways on major or collector streets). This “design vehicle” had problems with the aforementioned geometry, suggesting that the ITE drive- way design recommendations did not accommodate low-clearance vehicles. A similar statement can be made about the AASHTO standard railroad-highway grade crossing since French, Clawson, and Eck (5-25) found that car carrier trailers would hang-up on this crossing. Thus, additional research was conducted to develop driveway vertical alignment guidelines to accom- modate selected design vehicles. Operational Control A research team made measurements at 31 driveways with visible scrapes from the undersides of vehicles, and then measured speeds and elapsed travel times for over 1500 vehicles observed turning right or left into a number of driveways. The speed and elapsed time studies were con- ducted at commercial driveways on built-up suburban (but not CBD) arterial multilane roadways with posted speeds of 40 and 45 mph. All of the roadways had either a raised median or a TWLTL. These data were collected at driveways with right-turn entry radii ranging from 13 to 19.5 feet, and an entry lane width of about 13 feet. Very few vehicles about to enter a driveway exceeded 20 mph at the locations at which speeds were measured. After crossing the driveway threshold, average speeds for vehicles turning left into the driveway were around 10 mph. Vehicles that had turned right into the driveways were slightly slower, with average speeds around 7 mph. The speeds of vehicles entering driveways with breakover sag grades up to 10.5% were close to the speeds of vehicles entering flatter driveways. Scrapes on the pavement surface, presumably from the undersides of vehicles, began to be com- mon with a sag breakover of around 10 percent, and a crest breakover of about 11 percent. Exhibit 5-67. Reported steepest allowed driveway grades. Normally, Use This in Most Situations Commercial Residential Sm al le st re po rte d Av er ag e La rg es t re po rte d Sm al le st re po rte d Av er ag e La rg es t re po rte d Sm al le st re po rte d Av er ag e La rg es t re po rte d Grade: maximum (+) uphill from road allowed 2.6 9.7 15 5 7.5 10 6 11 15 Grade: maximum (-) downhill from road allowed -5 -9.4 -15 -5 -7.8 -10 -6 -11.0 -15 NOTE: These values reflect survey responses from 1 local and 16 state transportation agencies. + uphill drivewa y - downhill roadway roadway driveway

Geometric Design Elements 71 Exhibit 5-68. Driveway vertical profile guidelines. Category Description of Common Applications* Vertical Profile Guidelines Suggestion Rationale STANDARD DRIVEWAYS Very high intensity Urban activity center, with almost constant driveway use during hours of operation. Refer to roadway design guidelines. These driveways are often built to the standards of and resemble public roads and streets. FOR BOTH Higher intensity Medium-size office or retail, such as community shopping center, with frequent driveway use during hours of operation. Limit the maximum driveway grade to +8% (except where a lesser grade is required, such as when crossing a sidewalk), and the maximum sag breakover without a vertical curve between the roadway cross slope and an uphill driveway grade to 9%. Limit the driveway profile maximum grade change without a vertical curve for: a crest to 10% and a sag to 9%. From observations of vehicles entering driveways with radii up to 20 ft and comparisons of Flatter (1.5-5%) and Moderate (6-9%) grades revealed (1) little difference between speeds and travel times of vehicles turning right; and (2) only slight differences between speeds and travel times of vehicles turning left. From measurements of 31 driveways with scrape marks, underside dragging became a problem at a crest of about 11%, and at a sag of about 10%. AND Medium intensity Smaller office, retail, or other sites with occasional driveway use during hours of operation. Apartment complexes May limit the sag to 7%. Due to trailers. Lower intensity Single family or duplex residential, other types with very low use. May not apply to rural residential. Limit the driveway profile maximum grade change without a vertical curve for: a crest to 10% and a sag to 9%. From measurements of 31 driveways with scrape marks, underside dragging became a problem at a crest of about 11%, and at a sag of about 10%. SPECIAL SITUATION DRIVEWAYS CBD Refer to the guidelines above for “Higher intensity” and “Medium intensity.” Building faces are close to the street. Farm or ranch; Field Limit the driveway profile maximum grade change without a vertical curve for: a crest to 10% and a sag to 7%. A mix of design vehicles; some may be very low volume. These driveways should accommodate trailers. Industrial Varies, depending on types of vehicles. If low-boy trailers are expected, then limit crest Driveways are often used by large vehicles. breakover grades without a vertical curve to 3.5%. Other Motels Limit the driveway profile maximum grade change without a vertical curve for: a crest to 10% and a sag to 7%. Travelers pulling a trailer may stay at a motel; therefore, motel driveways should accommodate trailers. NOTES: Additional information on which to assess ground clearance is in Chp 3. The sag clearance for trailers is based on Eck’s evaluation; truck+trailer clearances will vary. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category. The study led to the suggestions following in Exhibit 5-68. Except where noted, these guide- lines are based on observations of passenger vehicles (P-vehicle). Where low-clearance vehicles are expected to traverse crest curves, refer to Exhibit 5-69 devel- oped by Eck and Kang (5-26) that suggests vertical curve lengths for various breakover angles (i.e., algebraic difference in grades). Drainage Control Surface runoff from the roadway should not inundate the sidewalk or spill over onto private property. It is also undesirable for the depth of flow to cover the driveway, making it difficult for motorists to determine were the edges of the driveway are.

72 Guide for the Geometric Design of Driveways There are a number of possible design scenarios, based on combinations of curbed or uncurbed roadways with driveway profiles that extend uphill or downhill from the connection with the roadway. Among the tools to combat surface runoff are driveway profile, driveway cross slope, drainage inlets near the driveway area, and drainage grates in the driveway. Exhibit 5-70 shows how profile design can be used to prevent water in the gutter from flowing onto private property. Roadway-Driveway Threshold Treatment The threshold is the edge or line where the roadway and the driveway join or touch. This line is often at the curb edge. Design concerns in this area include ease of travel for users (e.g., bicycles and motor vehicles), ease of construction, and, in cases where the roadway has a curb and gut- ter, confining drainage to the gutter line. Exhibit 5-71 shows four common driveway threshold treatments where the roadway has curbs: rolled curb, vertical lip, counterslope, and continuous. Exhibit 5-72 suggests design guidelines for driveway threshold treatments. The Continuous design is the preferred method. Except for single-family or duplex access on lower volume, lower speed residential streets, designers should avoid designs that create a bump at the threshold. Even in the single-family context, consider that a vertical discontinuity can be an impediment for bicyclists as well as pedestrians with disabilities (especially using wheelchairs). Vertical lip design is another topic needing additional research to assess the ability of other treat- ments to address drainage, to assess the detrimental effects of a pronounced lip, and to determine whether a low lip, perhaps on the magnitude of 1⁄2 inch, has any detrimental effects on users. For any type of treatment, the curb and gutter should not be broken off to leave a ragged edge, but should be cut with a saw and cleanly removed. Exhibit 5-73 shows a gutter treatment used in some jurisdictions. The gutter cross slope is sig- nificantly greater than that of the adjacent traveled lanes. This treatment is believed to improve drainage; however, it also increases the profile breakover angle which motorists entering and Exhibit 5-69. Minimum length of Type-II crest vertical curve to accommodate low-clearance vehicle. Algebraic Difference (%) Curve Length ft(m) 1 4 (1.2) 2 8 (2.4) 3 12 (3.7) 4 16 (4.9) 5 20 (6.1) 6 24 (7.3) 7 28 (8.5) 8 32 (9.8) 9 35 (10.7) 10 39 (11.9) Exhibit 5-70. Confining surface runoff flow. Roadway with curb: Setting the driveway profile with a crest vertical curve to slope down to the gutter, to confine the flow. ground ground driveway driveway roadway Roadway without curb: The driveway will (b)(a) need a crown, cross slope, or a grate in the sag to provide surface drainage. roadway shoulder

Geometric Design Elements 73 Exhibit 5-71. Driveway threshold treatment types. near-vertical lip at the gutter line STREET CROSS SECTION - DRIVEWAY PROFILE VIEW VERTICAL LIP cu rb curb shape does not change at a driveway STREET CROSS SECTION - DRIVEWAY PROFILE VIEW ROLLED CURB street street street streetcurb curb curb driveway (may slope up or down) driveway driveway (may slope up or down) (may slope up or down) driveway (may slope up or down) (b)(a) (d)(c) incline (steeper than driveway grade) behind the gutter line STREET CROSS SECTION - DRIVEWAY PROFILE VIEW COUNTERSLOPE no abrupt vertical component; driveway grade connects at gutter line STREET CROSS SECTION - DRIVEWAY PROFILE VIEW CONTINUOUS Exhibit 5-72. Driveway threshold treatment guidelines. Method Advantages and Disadvantages Comments Rolled curb Easiest threshold to construct, because the existing curb is not modified or removed. Confines the gutter flow, since the existing curb remains intact. Vehicles entering or exiting the driveway experience a jolt while crossing a curb of typical height. This method is generally unsuitable. It may be acceptable for single-family or duplex access on lower volume, lower speed residential streets. Vertical lip Construction requires curb modification or removal. Can confine very low flows in the gutter and reduce the spread of the gutter flow. Bump created by the vertical lip is a minor impediment to automobile movements and a more significant problem for turning bicyclists (i.e., bicycle tire strikes the face at a skew angle). Is often constructed by forming the threshold with lumber that leaves a vertical face or lip of 1 to 2 inches at the threshold. Counterslope Construction requires curb modification or removal. Can confine very low flows in the gutter and reduce the spread of the gutter flow. Less abrupt to cross, but can still be disruptive to automobiles and bicycles. The proportion and amount of rise and run affect the degree of disruption to automobiles and bicycles. Continuous OR Smooth Construction requires curb modification or removal. Is more bicycle- and automobile-friendly. If the driveway immediately slopes downward With this method, the profile slopes continuously but not abruptly upward from the gutter line. Thus the drainage objective can be suitably achieved by means that do not create problematic bumps for bicyclists or drivers. from the gutter line, this does not confine the drainage as well.

74 Guide for the Geometric Design of Driveways exiting the driveway have to negotiate. Many scrape marks on the driveway surface from the dragging of vehicles bumpers are clearly visible. More study should be done on this type of design to weigh any drainage benefits against impediments to traffic flow. Vertical Alignment Examples The following examples apply some of the guidelines for designing the vertical alignment of driveways. Exhibit 5-74 shows the driveway profile rising from the gutter line up to the sidewalk, then flattening at the sidewalk before falling as the driveway continues onto the private property. This type of design will confine normal depths of water in the gutter and not allow water to flow on to private property and down the driveway. Exhibit 5-75 shows the suggested values for driveways at which the P-vehicle is the design con- trol. If the near edge of the sidewalk is 5.5 feet from the face-of-curb or gutter line, and the drive- way is on a +7.0% grade, then the near edge of the sidewalk is 0.39 feet above the elevation of Exhibit 5-73. Increased gutter cross slope. Exhibit 5-74. Schematic showing driveway vertical alignment concepts. Normal curb location Driveway maximum 2.0% roadway sidewalk Exhibit 5-75. Example of a driveway vertical profile design. Driveway maximum 8.0% maximum 2.0% roadway Maximum breakover sag = 9% sidewalk Maximum breakover crest = 10% * Maximum breakover is the maximum without a vertical curve. - ELSE - Vertical curve

Geometric Design Elements 75 the gutter line. The 5-ft-wide sidewalk has a +2.0% cross slope, for a rise 0.1 foot, for total rise of 0.49 feet above the gutter line elevation. Exhibit 5-76 shows a design for a situation where the driveway would normally slope immedi- ately downward from the gutter line at a 4.33 percent grade. The alternate design (dashed line) allows the driveway to slope up from the gutter before sloping back down. Again, this design confines nor- mal depths of flow to the gutter, instead of allowing the gutter flow to rush down the driveway. Other Elements This section discusses other aspects of driveway design, such as landscaping, right-turn lanes on the roadway in advance of the driveway, surface drainage in the area where the driveway meets the roadway and sidewalk, use of traffic control device (e.g., signs, pavement markings, and traf- fic signals), and other situations. Landscaping and Business Signs Appropriate landscaping near roadway-driveway-sidewalk intersections can produce envi- ronmental and aesthetic benefits. Landscaping can also directly or indirectly help meet some geometric design objectives for one or more user groups. Landscaping can benefit driveway users in the following ways: • Landscaping helps reduce stormwater run-off and soil erosion. • Tree canopies can provide shade for pedestrians. • Trees that shade pavement can reduce asphalt temperatures by as much as 36°F and fuel tank temperatures by nearly 7°F (5-27). • Well-designed landscaping can help define driveway edges and make the driveway location more conspicuous. However, ill-chosen or ill-placed landscaping can be an inconvenience or even a hazard. Tree selection and suitability should consider climate, maintenance requirements, susceptibil- ity to disease, space available for root growth, ultimate tree height, and size of mature canopies. In more extreme cases, vegetation may physically interfere with one or more driveway user groups. Continuous maintenance of landscaping is essential to preserve plantings and sight lines, so the implications of budget limitations for maintenance should also affect landscaping decisions. Exhibit 5-77 presents suggested guidelines for the placement and control of vegetation (5-28). Exhibit 5-76. Example design for a downhill situation. When driveway slopes down from the gutter: (1) water in gutter flows into and down the driveway; (2) vehicles pulling into the roadway must overcome the grade. 15 ft @ 5.0% 45 ft @ 4.33% 16 ft vertical curve 30 ft @ 9.0% sidewalk in flat part of curve May need a vertical curve roadway gutter With the alternate design: (1) flow is confined to gutter, up to a depth of 0.47 ft; (2) vehicles entering the street depart from a downhill platform.

76 Guide for the Geometric Design of Driveways Business signs may be present outside of the roadway right-of-way, along driveways, or within parking areas. These signs should be placed so that they do not compete with traffic signs or obstruct sight lines of the various users. Along a busy roadway, a business sign may help identify a driveway location. If placed close to the driveway, a sign can help motorists who are scanning the upcoming roadside to detect the location of the driveway they are searching for. Conversely, a business sign located far from the driveway may actually divert a motorist’s view from the driveway location and be misleading and confusing. Auxiliary Right-Turn Lanes Right-turn deceleration lanes are frequently constructed to remove the slower right-turning vehicles from the through travel lanes when right-turn volumes into a driveway are heavy and/or could have a significant adverse effect on through traffic. The benefits that accrue from having right-turn lanes include increased capacity, reduced speed differentials and brake applications, and reduced rear-end collisions. Exhibit 5-77. Landscaping guidelines for driveways. Concern or Issue Design Response Specific Procedure and/or Information Provide unobstructed lines-of-sight among bicyclists, pedestrians, and vehicles in the area Do not install landscaping that blocks needed sight lines. Trees should be set back a sufficient distance from the driveway – public road intersections to avoid obstructing sight lines. In urban settings, trees generally should be set back at least 20 to 30 ft on the approach to intersections and 10 to 20 ft on the far side. However, in higher speed environments, greater setbacks may be required (5-29). Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed distance. The top of ground cover in driveway and street medians should not exceed 2 feet. This is 18 in. below the clear sight line of 42 in. The bottom of the tree canopy should be at least 5 ft (60 in. high). Landscaping should not obscure or interfere with traffic control devices or other roadside fixed objects Vegetation should be sufficiently removed from traffic signs. Vegetation should be sufficiently removed from utility lines. Landscaping should not create conflicts in the paths of users Limbs or branches that overhang any pedestrian path should be at least 7 ft above the surface of the path. Vegetation should be sufficiently removed from pavement surfaces to prevent roots from damaging sidewalks. Vegetation should be sufficiently removed from pavement edges to avoid scraping vehicles. Planting arrangements should not create concealed spaces. The ADA requires at least an 80 in. clearance above the pedestrian path (5-30). Landscaping should not interfere with adequate artificial illumination Trees should be set back at least 40 ft from luminaries. Preserve an adequate roadside clear zone Along major highways, the clear zone should normally extend at least 10 ft beyond the edge of the shoulder. See AASHTO Roadside Design Guide (5-31). More study is needed to better define the needed clear zone in lower speed, built-up urban street environments.

Geometric Design Elements 77 When a pedestrian is crossing a driveway, a right-turn auxiliary lane on the public highway allows a driver to wait without blocking a through traffic lane. A right-turn lane also removes turning vehicles from the through traffic lane, thus limiting interference with traffic progres- sion through a coordinated traffic signal system. Right-turn lanes may be desirable, but, where provided, should not be continuous, to avoid additional conflicts that would be introduced with both vehicular and bicycle traffic. Installation Guidelines The decision to provide an auxiliary right-turn deceleration lane on the roadway approach to a driveway intersection is usually made by the governing transportation agency. Although the driveway designer may not be in a position to make a decision as to whether a right-turn decel- eration lane should be installed, it is important for the designer to have some background information as to how a decision is made and on how provision of a right-turn deceleration lane may affect the driveway. Considerations in the decision making process generally include roadway volumes and speeds, driveway volumes, right-turn volumes, type of traffic control at the driveway intersection, and property availability. Some states have established application and design criteria for right-turn deceleration lanes for driveways and intersections on roadways under their jurisdiction, but the criteria vary widely from state to state (5-32)—two examples follow. Colorado DOT has warrants for right-turn decelerations based on roadway classification and posted speed (5-33). For example, on a roadway classified as an Expressway, Major Bypass (Category E-X), a projected peak-hour right-turn ingress turning volume greater than 10 vph would warrant a right-turn lane. For a Non-Rural Arterial (Category NR-A), a right-turn lane would be warranted for any access with a projected peak-hour right-turn ingress turning vol- ume greater than 50 vph; if the posted speed is greater than 40 mph, a right-turn deceleration lane would be warranted for any access with a projected peak-hour right-turn ingress turning volume greater than 25 vph. Florida DOT has guidelines based on posted speed and volume (5-9, p.60). For roadways with a posted speed of greater than 45 mph, 35 to 55 or more right turns per hour would war- rant a right-turn lane. For roadways with a posted speed of less than or equal to 45 mph, 80 to 125 or more right turns per hour would warrant a right-turn lane. The lower thresholds would be most appropriate on higher volume roadways or on two-lane roadways where lateral move- ment is restricted. The research in NCHRP Report 420 may be applied to assess the effects of right turns on curb lane operations (5-12). The installation of a right-turn deceleration lane has implications in terms of potential con- flicts with pedestrian movements. The objective of NCHRP Project 3-89, “Design Guidance for Channelized Right-Turn Lanes,” is to develop design guidance for channelized right-turn lanes, based on balancing the needs of passenger cars, trucks, buses, pedestrians (including pedestrians with disabilities), and bicycles. Design Considerations An auxiliary lane for either right- or left-turn lanes should be at least 10 feet wide and, ideally, should equal that of the through lanes. If the lane has curbs, the curb face should be appropri- ately offset from the lane edge (5-1). As shown in Exhibit 5-78, the length of the auxiliary lanes for turning vehicles consists of three components: entering taper, deceleration length, and storage length (5-1). Ideally, the total length of the auxiliary lane should be the sum of the length for these three components; however,

78 Guide for the Geometric Design of Driveways common practice is to accept a moderate amount of deceleration within the through lanes and to consider the taper length as part of the deceleration length. The following paragraphs summarize each component of the auxiliary lane length, based on information in the AASHTO A Policy on Geometric Design of Highways and Streets (5-1). Additional information is available in this AASHTO document. Taper. On high-speed rural roadways, a common practice has been to use a taper rate between 8:1 and 15:1 (longitudinal-to-transverse). In urban areas, some use a standard taper ranging in length from 50 to 100 feet. The following numbers provide an example from an agency that varies the taper length according to the posted speed: Posted speed (mph) 30 35 40 45 50 55 Straight line ratios 6:1 8:1 10.5:1 12.5:1 14.5:1 16.5:1 Source: New Mexico DOT, State Access Management Manual, Ch. 8, Sec. 18, p. 92, Sep. 2001 Some considerations favor shorter tapers over longer tapers at urban intersections, including driveways: 1. Shorter tapers appear to produce better “targets” for approaching drivers and to give more positive identification to an added auxiliary lane. 2. A longer taper may cause some drivers to incorrectly think that the deceleration lane is a through lane, especially when the taper is on a horizontal curve. 3. For the same total length of taper and deceleration, a shorter taper allows the storage length to be longer. This results in a longer length of full-width pavement for the auxiliary lane. The additional storage length helps to avoid turning traffic backing up in the through travel lanes and the slower speeds during peak periods would have a shorter taper needed. However, at higher vehicle speeds, this would involve deceleration in the through or turn lane. 4. During peak periods, when the queue length in the auxiliary lane is longer, speeds may decrease, which will make a shorter taper adequate. Deceleration Length. Provision for deceleration clear of the through traffic lanes is desirable on arterial roadways. Exhibit 5-79 lists turn lane deceleration distances from different sources. The braking distance component of stopping sight distance is included for comparison. On many urban facilities, an auxiliary turn lane is not long enough to accommodate the stor- age and all of the deceleration within its limits. Therefore, the initial part of the deceleration takes place in the through lanes, before the vehicle enters the auxiliary lane. In some higher volume and speed environments, significant deceleration in the through lanes may affect safety and operations adversely, so deceleration in the through lanes should be minimized. For steep upgrades, a shorter deceleration length may be acceptable. For significant down- grades, the deceleration distances need to be extended. Exhibit 5-78. Parts of a deceleration lane.

Geometric Design Elements 79 Storage Length for Left-Turn Lanes. At unsignalized driveway intersections, the storage length, may be based on the number of turning vehicles likely to arrive in an average 2-min period within the peak hour. Storage for at least two passenger vehicles should be provided. Where trucks represent more than 10 percent of the traffic, storage should be sufficient for at least one car and one truck. Storage Length for Right-Turn Lanes. At unsignalized driveways, if the turn lane does not stop or yield to other motor vehicles, and pedestrians seldom cross the driveway, no storage may be needed.If pedestriansoften cross the driveway, then storage for at least one vehicle may be desirable. Drainage of Surfaces Occupied by User Groups When there is deep standing or flowing water, the following undesirable scenarios can occur: • A passing motor vehicle will splash nearby bicyclists, pedestrians, or persons waiting at a tran- sit stop. In the more extreme cases, it may adversely affect a driver’s ability to control a vehicle. • Bicyclists and pedestrians are forced to wade through the water. A good driveway design considers and accommodates the flow of water that results from sur- face runoff in a way that minimizes inconvenience to users. Surface runoff water should flow toward a gutter, an inlet, a flume, a ditch, or other suit- able destination and not stand and pond in the roadway-driveway-sidewalk intersection area. Although it may be impossible to totally eliminate runoff, depths can be minimized and flows directed away from pedestrian users, the most vulnerable of the user groups. To achieve suitable drainage and avoid creating problems, the designer should examine the amount of and direction of surface flows in and near the intersection of the driveway with the roadway and sidewalk. The designer should specify the elevations of the surfaces of the driveway, the sidewalk, and the border on the design sheets. Drainage grates in the driveway can help intercept the surface runoff. As Exhibit 5-80 shows, if installed, grates do need to be inspected and maintained periodically to avoid creating poten- tially hazardous situations. Exhibit 5-79. Example deceleration lengths. Deceleration Length (ft) Design Speed AASHTO AASHTO Fla Fla NM NM Wis (mph) Brakinga Value Comfortableb Value (Urban)c Values (Rural)d Stope 15mphf d2g 30 86 170 -- -- 200 175 160 35 118 -- 145 -- 250 230 -- 40 154 275 155 -- 325 300 275 45 194 340 185 -- 400 370 -- 50 240 410 240 290 475 450 425 55 290 485 -- 350 550 525 -- -- indicates no value given for this speed a. braking distance (2004, p. 112) b. deceleration (2004, p. 714) c, d. assumes 10 mph speed difference (from Std. Index 301, rev. 2005). The FDOT Driveway Handbook (2005, p. 63) says “Right turn lane tapers and distances are identical to left turn lanes under stop conditions.” e, f. New Mexico State Access Mgmt. Manual, Ch. 8, Sec. 18, p. 92, Sep. 2001. One condition is deceleration to 0 mph, the other is to 15 mph. g. Wisconsin Facilities Development Manual, 11-25-1, p. 1-3, May 2006. Distance d2 is the distance traveled while the driver maneuvers laterally and stops. The values allow a 10 mph speed difference when the turning vehicle clears the through lane.

80 Guide for the Geometric Design of Driveways Designing to avoid directing roadway gutter flow into a driveway and onto private property was discussed in the Vertical Alignment section. Traffic Controls Signs, pavement markings, and traffic signals are called traffic control devices (TCDs). Strictly speaking, they are not geometric design elements, but TCDs may be used to complement a geo- metric design. Because of low volumes and speeds, TCDs are not needed on many driveways. Driveways with moderate to high traffic volumes are more likely to need some form of traffic control, such as signs and/or pavement markings. Where TCDs are installed, they should be consistent with the signs and markings that motorists and pedestrians are familiar with, the ones they see on the surrounding roadway sys- tem. The Manual on Uniform Traffic Control Devices (MUTCD) (5-34) for streets and highways sets forth the guidelines for the application of traffic signs, pavement markings, signals, and other TCDs. Sign Considerations Among the many situations that call for signs, the following are likely to be found at driveways and perhaps overlooked by some designers: • Along an undivided roadway, when triangular islands (pork chops) are constructed in the driveway entry throat to prohibit one or both left turns, installation of No Left Turn (R3-2) sign(s) in conformance with the MUTCD is needed. • If a driveway has a wide median, drivers may find that the R4-7 Keep Right (of the median nose) sign is helpful. • Driveways intended for one-way operation should be accompanied by appropriate signs, so motorists will not proceed in the wrong direction. Refer to the MUTCD for information about the use of One-Way, turn prohibition, and Do Not Enter signs. If a driveway connects with a narrow roadway, motorists may find parked cars make turn- ing into or out of the driveway difficult or impossible. Some situations may call for parking Exhibit 5-80. Damaged grates in driveway need repair.

Geometric Design Elements 81 prohibitions in advance of and past the connection and on the other side of the roadway oppo- site the connection. Marking Considerations The MUTCD requires that pavement markings separating opposite directions of travel, such as a center line, be yellow. Markings separating the same direction of travel (e.g., lane lines) and the outer edge lines are white, as are stop lines, crosswalk markings, and directional turn arrows. Where driveways are wide enough to accommodate three or more lanes of traffic, pavement markings to delineate the intended lanes should be provided. Exhibit 5-81 shows two drive- ways wide enough for three lanes of traffic. On the driveway without the pavement markings, motorists are much more likely to position their vehicles so as to create problems and conflicts with other vehicles. Some driveways with multiple exit lanes are marked with slightly offset stop lines, as shown in Exhibit 5-82. This is done so that when both left-turning and right-turning vehicles are try- ing to exit the driveway at the same time, the left-turning vehicle does not block the needed line- of-sight of the right-turning driver. The right-turning vehicle is given preference because a safe right-turn maneuver requires only an adequate size gap from the left, while a safe left-turn maneu- ver requires adequate size gaps from both the left and the right. This offset also accommodates the path of a vehicle turning left from the roadway into the driveway. Channelizing devices, such as tubular markers, have been used to enhance delineation and to reinforce turn prohibitions. They are sometimes part of a driveway triangular island installation. The MUTCD provides detailed instructions for using channelization devices, including that, if used at night, they are to be retroreflective. Exhibit 5-83 shows tubular markers used to discour- age unwanted left turns. Signal Considerations Where high-volume driveways intersect public roadways, traffic signals may be necessary. Considerations are listed in Exhibit 5-84. Driveways sometimes essentially form the fourth leg of a signalized intersection. The current MUTCD does allow a driveway that forms the fourth approach or leg of an otherwise signalized intersection to be unsignalized. Before deciding to exercise this option, the designer should ascer- tain that volumes and speeds on the other three approaches as well as on the driveway are low enough so that vehicles from the unsignalized driveway can safely enter the intersection. Exhibit 5-81. Wider driveways without and with pavement markings. (a) (b)

82 Guide for the Geometric Design of Driveways Railroad Grade Crossings Where there is a practical alternate route, it is desirable that a driveway not cross railroad tracks at grade. However, in some cases, the only access to a public road may be across a rail- road track. The guidance in the Railroad-Highway Grade Crossing Handbook (5-35) for design of rail- highway crossings also applies to the design of driveway-rail crossings. Exhibit 5-85 lists some design considerations for driveways crossing railroad tracks. Track maintenance can result in raising the track as new ballast is added to the track struc- ture. The Handbook cautions that “unless the highway profile is properly adjusted, this practice will result in a ‘humped’ profile that may adversely affect the safety and operation of highway traffic over the railroad.” The greatest risk of becoming hung up at railroad-highway grade crossings because of contact with the track or highway surface is posed by low-clearance, long- wheelbase vehicles. A similar problem can occur where the crossing is in a sag vertical curve. In this case, the front or rear overhangs on certain vehicles can strike or drag the pavement. When a road parallels a railroad and an intersecting driveway crosses the railroad, a rail- road grade crossing near the roadway intersection results. The Railroad-Highway Grade Exhibit 5-83. Example of tubular markers to prohibit a movement. Exhibit 5-84. Traffic signal considerations. Traffic Signal Design Element Suggested Practice Minimum green time To accommodate pedestrians crossing either the driveway or the roadway, the designer can either establish a minimum green time that is adequate for crossing or provide pedestrian pushbuttons. Users with disabilities Many situations, especially in urban areas, call for features such as detectable warnings to accommodate pedestrians with disabilities. Actuation In most cases, want an actuated signal, so will not take away green from the through roadway unless there is actual driveway demand. Semi-actuated may be adequate. Progression If the driveway traffic signal is one of a series along the roadway, then time and coordinate the signal to minimize interference with progression along the through roadway. Semi-actuated traffic signals may help minimize interruptions to through traffic on the public road. Exhibit 5-82. Offset stop line markings. yellow white roadway dr ive wa y sidewalk

Geometric Design Elements 83 Crossing Handbook points out that the higher occurrence of collisions at these intersections is due in part to a short storage area for vehicles waiting to move through the crossing and the intersection. “If the intersection is signalized or if the driveway approach from the cross- ing is controlled by a STOP sign, queues may develop across the crossing, leading to the pos- sibility of a vehicle becoming ‘trapped’ on the crossing. Also, there are more distractions to the motorist, leading to the possibility of vehicle-vehicle conflicts.” The critical distance between a driveway-rail crossing and a driveway-highway intersection is a function of the number and type of vehicles expected to be queued up by the intersection traffic control. If other viable driveway locations are available, consider routing the driveway so that it does not cross the railroad track. References 5-1. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) 896 pp. 5-2. AASHTO. Guide for the Planning, Design, and Operation of Pedestrian Facilities. Washington, DC (July 2004) 127 pp. 5-3. Louisville-Jefferson County Metro Government. Title VII: Traffic Code, Chapter 74: Bicycles and Motorcycles, Section 74.01: Operation of Bicycles (2008) no page numbers. 5-4. Guth, D., and LaDuke, R. “Veering by Blind Pedestrians: Individual Differences and Their Implications for Instruction.” Journal of Visual Impairment and Blindness (1995) pp. 28–37. 5-5. Guth, D., and Rieser, J.J. “Perception and the Control of Locomotion by Blind and Visually Impaired Pedestrians,” in Blasch, B.B., Wiener, W.R., & Welsh, R.L. (eds.) Foundations of Orientation and Mobility (2nd ed.), AFB Press, New York (1997) pp. 317–356. 5-6. Fitzpatrick, K., et al. TCRP Report 19: Guidelines for the Location and Design of Bus Stops. Transportation Research Board, National Research Council, Washington, DC (1996). 5-7. Gattis, J. L. “Class Notes, CVEG 4423,” Univ. of Arkansas, Fayetteville, AR (2009). 5-8. Highway Capacity Manual. Transportation Research Board, National Research Council, Washington, DC (2000). 5-9. Systems Planning Office. Driveway Handbook. Florida Department of Transportation, Tallahassee, FL (March 2005) 100 pp. 5-10. Box, P.C. “Driveway Accident and Volume Studies, Part I-General Relationships.” Public Safety Systems (May/June 1969) pp. 18–22. Exhibit 5-85. Considerations for driveways crossing railroad tracks. Design Element Suggested Practice Reason Intersection angle Driveway and railroad tracks intersect at 90 degrees Enhance the driver’s view of the crossing Horizontal alignment - curvature Crossings should not be located on either driveway or railroad curves Driveway curvature limits a driver’s view of the crossing ahead and the driver’s attention may be directed toward negotiating the curve rather than looking for a train. Railroad curvature restricts a driver’s view down the tracks from both a stopped position at the crossing and on approach to the crossing Vertical alignment Driveway-rail crossing should be as level as possible. AASHTO (5-1, pp. 731-733) recommends that the crossing surface be in the same plane as the top of rails for a distance of 2 feet outside the rails, and that the surface of the roadway be not more than 3 inches higher or lower than the top of the nearest rail at a point 30 feet from the rail, unless track superelevation dictates otherwise. Improved sight distance, rideability, and braking and acceleration distances

5-11. Neuman, T.R. NCHRP Report 279: Intersection Channelization Design Guide. Transportation Research Board, National Research Council, Washington, DC (1985) p. 76. 5-12. Gluck, J. S., Levinson, H. R., and Stover, V. G. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, National Research Council, Washington, DC (1999) 157 pp. 5-13. Stover, V. G., and Koepke, F. J. Transportation and Land Development, 2nd edition. ITE, Washington, DC (2002) 700 pp. 5-14. Gattis, J.L., and Low, S.T. Transportation Research Record 1612, “Intersection angle geometry and the driver’s field of view.” Transportation Research Board, National Research Council, Washington, DC (1997) pp. 10–16. 5-15. Son, Y. T., Kim, S. G., and Lee, J. K. Transportation Research Record 1796, “Methodology to calculate sight distance available to drivers at skewed intersections.” Transportation Research Board, National Research Council, Washington, DC (2002) pp. 41–47. 5-16. Garcia, A. “Lateral vision angles and skewed intersection design.” Proceedings, Third International Symposium on Highway Geometric Design (2005). 5-17. Center for Transportation Research and Education. Access Management Toolkit: Answers to frequently asked questions. Iowa State University, Ames, IA, http://www.ctre.iastate.edu/Research/access/toolkit/ (as of Dec. 31, 2007). 5-18. Missouri Department of Transportation. New Engineering Policy Guide, 940.16. Jefferson City, MO, http://epg.modot.org/index.php?title=940.16_Driveway_Geometrics (as of Dec. 31, 2007). 5-19. New Hampshire Department of Environmental Services. Innovative Land Use Planning Techniques: A Handbook for Sustainable Development (draft). Concord, NH, http://www.des.nh.gov/REPP/ilupth/ Access_Management.doc (as of Dec. 31, 2007). 5-20. Koepke, F.J., and Levinson, H.S. NCHRP Report 348: Access Management Guidelines for Activity Centers. Transportation Research Board, National Research Council, Washington, DC (1992) pp. 94–95. 5-21. Committee on Access Management. Access Management Manual. Transportation Research Board, National Research Council, Washington, DC (2003) pp. 184–185. 5-22. City of Roseville, CA. “Design Standards,” (March 2007) p. 13. http://www.roseville.ca.us/civica/filebank/ blobdload.asp?BlobID=2387 (as of Oct. 28, 2008). 5-23. Kulash, W.M. Residential Streets, 3rd edition. Urban Land Institute, National Association of Home Builders, ASCE and ITE, Washington, DC (2001) 76 pp. 5-24. Eck, R.W., and Kang, S.K. Transportation Research Record 1327, “Low-Clearance Vehicles at Rail-Highway Grade Crossings: An Overview of the Problem and Potential Solutions.” Transportation Research Board, National Research Council, Washington, DC (1991) pp. 27–35. 5-25. French, L.J., Clawson, A., and Eck, R.W. Transportation Research Record 1847, “Development of Design Vehicles for Hang-Up Problem.” Transportation Research Board, National Research Council, Washington, DC (2003) pp.11–19. 5-26. Eck, R.W., and Kang, S.K. Transportation Research Record 1356, “Roadway Design Standards to Accom- modate Low-Clearance Vehicles.” Transportation Research Board, National Research Council, Washington, DC (1992) pp. 80–89. 5-27. Scott, K.I., Simpson, J.R., and McPherson, E.G. “Effects of Tree Cover on Parking Lot Microclimate and Vehicle Emissions.” Journal of Arboriculture, Vol. 25, No. 3 (1999) pp. 129–142. 5-28. Eck, R.W., and McGee, H. Vegetation Control for Safety: A Guide for Local Highway and Street Maintenance Personnel. FHWA-SA-7-18, FHWA, Washington, DC (July 2007) pp.13–6, 21–23. 5-29. Schellinger, D. In Planning and Urban Design Standards. Sendich, G., Graphics Editor, American Planning Association, John Wiley and Sons, New York (2006). 5-30. ADA Accessibility Guidelines for Buildings and Facilities (ADAAG), September 2002, http://www. access-board.gov/adaag/html/adaag.htm#4.4 5-31. AASHTO. Roadside Design Guide, 3rd Edition. Washington, DC (2002) 344 pp. 5-32. “Lane Widths, Channelized Right Turns, and Right-Turn Deceleration Lanes in Urban and Suburban Areas.” Contractor’s Final Report NCHRP Project 03-72, Transportation Research Board, National Research Council, Washington, DC. 5-33. Colorado Transportation Commission. Colorado State Highway Access Code. Colorado DOT, Denver, CO (1998, revised March 2002). 5-34. FHWA. Manual on Uniform Traffic Control Devices for Streets and Highways. Washington, DC (2003) 760 pp. 5-35. Ogden, B.D. Railroad-Highway Grade Crossing Handbook – Revised Second Edition 2007. ITE, Washington, DC (2007) 324 pp. 84 Guide for the Geometric Design of Driveways

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 659: Guide for the Geometric Design of Driveways explores guidelines related to the geometric design of driveways. The report includes driveway-related terms and definitions, an examination of basic geometric controls, a summary of access spacing principles, and detailed discussions of various geometric design elements.

Material related to and supporting the contents of NCHRP Report 659, including an extensive review of literature, has been published as NCHRP Web-Only Document 151: Geometric Design of Driveways.

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