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Designers use a design vehicle or design user to set characteristics of the roadway, transitway, sidewalk, and cycleway. Designing for comfortable use by the occasional large truck often results in overly wide roads or high-speed turns by cars, and opportunities are lost to create space for other, morefrequent users such as pedestrians. Select a design vehicle—a routine user for whom the street is designed—as well as a control vehicle—one that only occasionally uses the street — to prevent the overdesign of a facility. Safe design means tailoring elements for the most vulnerable street user rather than the largest possible vehicle.
The design vehicle is the least maneuverable vehicle that routinely uses a street or a facility. This could be a pedestrian in a wheelchair, a cyclist on a cargo bike, a delivery truck, or a transit bus, depending on the type of facility and its user volumes. The choice of design vehicle directly affects the design of the street, impacting the safety and comfort for each user. In particular, intersections and lane transitions are designed for comfortable use by the design vehicle.
The control vehicle is the least maneuverable vehicle that is ever planned to use a street, but potentially at very low speeds or with multipoint turns.
Use both a design vehicle and a control vehicle to determine intersection turn radii and lane widths, understanding that the control vehicle is an infrequent presence on the street and can be accommodated with temporary interventions such as flaggers or road closures, and may use multiple lanes and mountable street elements to make turns.1 Use advance stop bars or other elements to accommodate movements by design vehicles. Do not widen existing intersections to permit larger vehicles to turn.
For pedestrian facilities such as sidewalks, ramps, and crossings, key design vehicles include a person using a wheelchair or a small group of people walking together. In some cases, the design case will be two such groups passing one another. A large group, such as a class of children, can be used as a control vehicle, particularly for pedestrian islands.
For cycle facilities, use cargo bikes or, where present, cycle rickshaws as the design vehicle, especially when designing cycle lane curves, transitions and grade changes, as well as confined areas.
For transit facilities including transitways, transit lanes, and mixedtraffic lanes with buses, use the typical transit vehicle as a design vehicle, but only for movements used by transit. Since transit routes do not turn at every intersection, but may use off-route streets to turn around, coordinate with transit operators to determine transit turn locations and design these turns accordingly.
For motor vehicles, choose the smallest turn radius and curve that accommodates routine or frequent use. Design for very low turning speeds of no more 10 km/h. Small corner radii at intersections also shorten pedestrian crossing distances and save signal time. See: Corner Radii.
Where emergency vehicles are much larger than the design vehicle, they can be permitted to make turns by using all areas of the right-of-way, including mountable corner islands or median tips, and portions of the sidewalk, where necessary. Flexible bollards, mountable curbs, and other devices facilitate emergency movements. Work with emergency responders to reduce the size or turn radius needed by newly purchased vehicles.
Larger vehicles may be restricted from certain roads based on context or street typology in order to allow the use of a smaller design vehicle. While restrictions on large vehicles are often made by necessity in central areas or historic districts, large vehicles may also be restricted to permit safer, human-scale street design on newer streets. Larger vehicles may be permitted at certain times, or deliveries may be made by hand truck or cargo cycle. These methods can prevent the choice of excessively large design vehicles. See: Operational and Management Strategies.
1. American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets (Washington, DC: AASHTO , 2011).
Adapted by Global Street Design Guide published by Island Press.
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