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Geometric Design Elements 29 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. Exhibit 5-7. Driveway design considerations related to bicyclists. Concern or Issue Design Response Specific Procedure and/or Information Bicyclists, motorists in Provide horizontal and vertical Refer to the latest edition of the vehicles, and pedestrians alignment that provides an AASHTO Green Book for the need to see each other far adequate advance view of the procedure to calculate the enough in advance to driveway intersection. needed stopping sight distance or avoid collisions. Do not place anything in the border intersection sight distance. that blocks needed sight lines. Abrupt change in cross Where a bicycle path or other slope causes bicyclists to similar route crosses a driveway, lose balance. do not have an abrupt change where the bike path cross slope meets the driveway grade. Abrupt change in surface Where a bicyclist could turn into or elevation causes turn out of a driveway, do not have bicyclists to lose control. an abrupt change in surface elevation that creates a bump for the bicyclist. Relatively thin bicycle Do not have any grate openings that tires are vulnerable to a bicycle tire could drop into. openings in the surface.
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30 Guide for the Geometric Design of Driveways Exhibit 5-8. Driveway design considerations related to pedestrians. Concern or Issue Design Response Specific Procedure and/or Information Bicyclists, motorists in Provide horizontal and vertical Refer to the latest edition of the vehicles, and pedestrians alignment that provides an AASHTO Green Book for the need to see each other far adequate advance view of the procedure to calculate the enough in advance to driveway intersection. needed stopping sight distance avoid collisions. Do not place anything in the border or intersection sight distance. that blocks needed sight lines. Also check sight for people of lower height, such as children or wheelchair users. An excessive sidewalk ADA requirements specify a The PAR requirement applies cross slope (or driveway pedestrian travel path (called a not only to the crossing of the grade) adversely affects Pedestrian Access Route or "PAR" driveway, but also to the the crossing by in the Draft Public Rights-of-Way sidewalk connections. pedestrians with vision Accessibility Guidelines) with a The combination of 2% impairments and those in cross slope that does not exceed maximum cross slope and the wheelchairs. 2%. different sidewalk location options affect the vertical alignment of the driveway in different ways. Is there a suitable The sidewalk alignment across the pedestrian route across driveway should be straight and the driveway? not have "steps" or other abrupt changes in vertical elevation. 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
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Geometric Design Elements 31 Exhibit 5-9. Sidewalk-driveway treatments. Exhibit 5-10. Unacceptable vertical curb where sidewalk crosses driveway. Exhibit 5-11. Driveway with very limited sight distance.