Elements of Complete Streets 37 practitioners to resources such as USLIMITS2 to re-evaluate speed limits within speed zones. It is anticipated the FDOT Greenbrook will then be updated to support this national change. Strategies to address speed when not rebuilding a road include various tools often referred to as traffic calming and speed management. VEHICLE LANE WIDTHS Narrow lanes have been shown to result in slower vehicle speeds. Lane widths have traditionally been wide (12 feet). Lane dieting, a reduction in lane widths, can be an effective way to repurpose space on streets which otherwise seem full. The AASHTO Green Book offers substantial flexibility on lane widths depending on the desired speed, capacity, and context of the road. Per FHWA guidance, ten-foot lanes are appropriate for urban environments where posted speeds are less than 45 MPH and “narrower lane widths may be chosen to manage or reduce speed and shorten crossing distances for pedestrians.” An 11-foot outside lane may be provided on designated truck or transit routes to allow for increased safety of all users and to improve the efficiency of bus operations. Saturation flow rates do not change for lanes between 10-12 feet according to the Florida Department of Transportation 12 . Lane diets should be considered during routine maintenance and resurfacing projects to increase the space available to other modes. DESIGN VEHICLE The design vehicle influences several geometric design features including lane width, corner radii, median nose design, and other intersection design details. Design vehicles are identified by AASHTO in four general classes including passenger vehicles, trucks, buses, and recreational vehicles. Figure 2-8 illustrates the different sizes and wheelbases for trucks and buses. Designing for a larger vehicle than necessary is undesirable, due to the potential negative impacts larger dimensions may have on pedestrian crossing distances and the speed of turning vehicles. On the other hand, designing for smaller vehicles can result in operational problems if larger vehicles frequently use the facility.
Figure 2-8 Different Bus and Truck Wheelbases (Seattle Streets Illustrated)
Figure 2-9 Turn Radii of City Bus (NACTO)
12 Florida Department of Transportation Conserve by Bicycle Report http://www.fdot.gov/safety/4-reports/Bike-Ped/CBBphase1%20Apps%20A-P.pdf
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