4.3.2 Design Vehicles

The key controls in geometric highway design are the physical characteristics and the proportions of vehicles of various sizes using the roadway. Therefore, it is appropriate to examine all vehicle types, establish general class groupings, and select vehicles of representative sizes within each class for design use. These selected vehicles, with representative weight, dimensions, and operating characteristics, are used to establish roadway design controls for accommodating designated vehicle classes and are known as design vehicles.
There are four general classes of design vehicles:
  • Passenger Cars
     include cars of all sizes, sport/utility vehicles, minivans, vans, and pickup trucks;
  • Buses
    include motor coaches, city transit, school, and articulated buses;
  • Trucks
    include single unit trucks, truck tractor semitrailer combinations, and truck tractors with semitrailers in combination with full trailers; and
  • Recreation Vehicles
     include motor homes, cars with camper trailers, cars with boat trailers, and motor homes with pulling cars.
When designing the roadway facility, the designer should consider the largest design vehicle that is likely to use that facility with considerable frequency.
If a larger class of vehicle typically uses the facility, that vehicle shall govern the design, regardless of the minimum design vehicle required for the type of facility.
A large school bus (84 passengers) should be utilized for the design of intersections of highways with low-volume county roads and township/local roads under 400 AADT along with some subdivision street intersections.
The WB-67 truck should generally be the minimum size design vehicle considered for intersections of freeway ramp terminals with arterial crossroads and for other intersections on State highways and industrialized streets that carry high volumes of truck traffic.
For more guidance on design vehicles and representations of design vehicles turning movements refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
and
NCHRP Report 505
.
Table 4-1
depicts the minimum turning radii for design vehicles.