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CHAPTER 5 Geometric Design Elements 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. 24

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Geometric Design Elements 25 Exhibit 5-1. Unacceptable driveway designs. Exhibit 5-2. Driveway geometric design considerations that may be within the control of the designer. 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: O O O flat-angle (turn angle 90 ) 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) (continued on next page)