Guide for the Geometric Design of Driveways (2010) / Chapter Skim
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Pages 24-84
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From page 24... ...
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)
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From page 25... ...
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)
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From page 26... ...
The basics of stopping sight distance and intersection sight distance are explained in the AASHTO Green Book (5-1, pp.
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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
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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.
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From page 29... ...
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.
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From page 30... ...
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 sidewalk 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.
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From page 31... ...
Geometric Design Elements 31 Exhibit 5-10. Unacceptable vertical curb where sidewalk crosses driveway.
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From page 32... ...
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 pedestrian crossing, or a delineating guide strip across the driveway width might mitigate this situation on a wide driveway.
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From page 33... ...
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.
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From page 34... ...
(c) 34 Guide for the Geometric Design of Driveways
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From page 35... ...
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.
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From page 36... ...
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)
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From page 37... ...
Types of driveway connection transition geometry. Curved radius drivewaydriveway drivewaydriveway THESE ARE PLAN (TOP)
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From page 38... ...
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)
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From page 39... ...
Category Description of Common Applications* Suggested Driveway Transition Shape Design (assuming curbed roadways in urban area, uncurbed in rural area)
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From page 40... ...
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.)
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From page 41... ...
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.
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From page 42... ...
Restrictive medians offer several safety benefits. Restrictive medians in the roadway provide refuge for pedestrians crossing the roadway.
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From page 43... ...
. Exhibit 5-30 shows how restrictive medians may be used to eliminate some or all of the leftturn movements in and out of driveways.
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From page 44... ...
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.)
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From page 45... ...
. 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.
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From page 46... ...
Florida DOT does not use driveway triangular islands on undivided roadways (5-9)
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From page 47... ...
along the middle of the public roadway. Inappropriate Channelization Although channelization can be beneficial, it can also be ineffective or inappropriate in some situations.
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From page 48... ...
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)
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From page 49... ...
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.
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From page 50... ...
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)
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From page 51... ...
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.
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From page 52... ...
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)
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From page 53... ...
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.
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From page 54... ...
Driveway edge clearance from fixed objects.
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From page 55... ...
Minimum length of the driveway throat or connection depth 4. Accommodating the need to reverse vehicle direction (i.e., turn around)
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From page 56... ...
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.
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From page 57... ...
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 intersecting roadway to the first point along the driveway at which there are conflicting vehicular traffic movements.
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From page 58... ...
• 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 themselves and detect conflicting traffic movements from crossroads, parking spaces, bicycle routes, or pedestrian paths they encounter.
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From page 59... ...
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.
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From page 60... ...
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 highvolume traffic generators, it is the exit condition that governs the needed throat length.
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From page 61... ...
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.
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From page 62... ...
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)
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From page 63... ...
Other Throat Lengths. For a comparison, Exhibit 5-59 presents minimum throat length criteria from two states, New Mexico and Florida.
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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)
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From page 65... ...
turnarounds may be used. Exhibit 5-61 offers minimum desirable circular turnaround dimensions (5-23)
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From page 66... ...
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 improvements 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.
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From page 67... ...
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.
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From page 68... ...
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 vehicle does not drag on the roadway or driveway surface.
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From page 69... ...
The design vehicle for vertical alignment may be different from the design vehicle used to design the horizontal alignment (e.g., turning radii)
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From page 70... ...
Thus, additional research was conducted to develop driveway vertical alignment guidelines to accommodate 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.
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From page 71... ...
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.
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From page 72... ...
, ease of construction, and, in cases where the roadway has a curb and gutter, 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.
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From page 73... ...
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)
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From page 74... ...
Example of a driveway vertical profile design. Driveway maximum 8.0% maximum 2.0% roadway Maximum breakover sag = 9% sidewalk Maximum breakover crest = 10% *
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From page 75... ...
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 traffic signals) , and other situations.
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From page 76... ...
. Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed distance.
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From page 77... ...
. 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)
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From page 78... ...
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.
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From page 79... ...
Exhibit 5-79. Example deceleration lengths.
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From page 80... ...
. Strictly speaking, they are not geometric design elements, but TCDs may be used to complement a geometric design.
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From page 81... ...
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.
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From page 82... ...
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.
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From page 83... ...
TCRP Report 19: Guidelines for the Location and Design of Bus Stops. Transportation Research Board, National Research Council, Washington, DC (1996)
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From page 84... ...
Transportation Research Record 1847, "Development of Design Vehicles for Hang-Up Problem." Transportation Research Board, National Research Council, Washington, DC (2003)
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