Design vehicle

Another important factor determining a roundabout’s layout is the need to accommodate the largest motorized vehicle likely to use the intersection. The turning path requirements of this vehicle, termed hereafter the design vehicle, will dictate many of the roundabout’s dimensions. Before beginning the design process, the designer must be conscious of the design vehicle and possess the appropriate vehicle turning templates or a CAD-based vehicle turning path program to determine the vehicle’s swept path.

The choice of design vehicle will vary depending upon the approaching roadway types and the surrounding land use characteristics. The local or State agency with jurisdiction of the associated roadways should usually be consulted to identify the design vehicle at each site. The AASHTO A Policy on Geometric Design of Highways and Streets provides the dimensions and turning path requirements for a variety of common highway vehicles (4). Commonly, WB-15 (WB-50) vehicles are the largest vehicles along collectors and arterials. Larger trucks, such as WB-20 (WB-67) vehicles, may need to be addressed at intersections on interstate freeways or State highway systems. Smaller design vehicles may often be chosen for local street intersections.

In general, larger roundabouts need to be used to accommodate large vehicles while maintaining low speeds for passenger vehicles. However, in some cases, land constraints may limit the ability to accommodate large semi-trailer combinations while achieving adequate deflection for small vehicles. At such times, a truck apron may be used to provide additional traversable area around the central island for large semi-trailers. Truck aprons, though, provide a lower level of operation than standard nonmountable islands and should be used only when there is no other means of providing adequate deflection while accommodating the design vehicle.

Exhibits 6-15 and 6-16 demonstrate the use of a CAD-based computer program to determine the vehicle’s swept path through the critical turning movements.