Design Vehicle

Article

Design vehicle is defined by ITE as the vehicle that must regularly be accommodated on a roadway without encroachment into other travel lanes. The design vehicle’s dimensions and movements can play a large role in the physical characteristics of a roadway, such as the appropriate lane width and the radii of curves at intersections and driveway corners.

AASHTO has developed several profiles for commonly-used design vehicles, the details of which are provided in Chapter 2 of AASHTO’s Policy on the Geometric Design of Highways and Streets. The profiled design vehicles range in size from passenger cars to interstate tractor-trailers. Larger design vehicles require larger roadway dimensions, particularly at intersections. 

There are a number of tradeoffs inherent in design vehicle selection. The design vehicle selected for a given roadway should represent the largest vehicle that regularly or frequently, not occasionally, uses it. Selecting too large a design vehicle for a roadway or roadway segment will result in wider lanes and intersections, jeopardizing safety for other modes and leaving less space for pedestrian, bicycle and transit infrastructure.

Selecting too small a design vehicle can make turning maneuvers difficult or impossible for larger vehicles, potentially causing congestion and/or safety issues. Balancing these tradeoffs is an essential component of creating a great street which adequately serves regular users and is appropriate for the place type.

Characteristics which influence design vehicle selection for Residential Neighborhoods:

• Large truck traffic is uncommon
• Transit vehicles are often present
• Significant pedestrian presence is likely

Residential areas generally should not be designed to accommodate large vehicles.  These types of areas are typically comprised of numerous small residential parcels, and they usually have significant pedestrian travel.  Choosing the smallest practical design vehicle (the largest vehicle that regularly uses the facility) for these place types can produce a number of benefits, including:

• Minimizes intersection and driveway “footprints” by minimizing the turning radii requirements at corners, leaving more space for adjacent land uses, including transit and pedestrian activity.
• The smaller radii also minimize the speed at which vehicles can turn around corners, improving pedestrian safety.
• Minimizes lane width requirements, which reduces the right-of-way required for vehicular traffic, leaving more space for transit, pedestrian activity, landscaping, and various roadside improvements.  Narrower lanes also result in a shorter crossing distance for pedestrians at intersection and mid-block crosswalks.

Credit: CH2M HILL

If buses and/or light rail lines are present in these areas, it is important to coordinate with regional transit agencies to determine the appropriate dimensions and considerations if a bus is to be the facility’s design vehicle.  Special attention is required if articulated buses are expected to use the thoroughfare, as these buses have different design requirements than standard buses.

Considerations for larger trucks:

Large trucks are commonly and perhaps even increasingly used for commerce and shipping. Ideally, large truck traffic should be rerouted to mobility-oriented arterials; however, there is still the need to accommodate large trucks delivering goods and materials to destinations along residential thoroughfares. In other instances, there may even be a local factory or other large truck generator in close proximity to the place in question.  In such cases, the following options should be considered:

1. Keep design dimensions such as curb return radii small, but offset sidewalks, light poles, street furniture, and other streetscape amenities, thus allowing the occasional larger vehicle to ride over the curb when negotiating the turn, if necessary.

2. Emergency response vehicles must be considered. Coordinate early and often with local fire and police departments to insure that the ultimate design considers their needs. Balancing tradeoffs and compromise on behalf of all stakeholders is essential for successful implementation.

3. Do not allow the occasional moving truck or delivery truck to dictate thoroughfare design. These vehicles should be expected to navigate with in the confines of a street designed for smaller vehicles. It may require crossing into another lane or riding up on a curb, but these are worthwhile tradeoffs in order to preserve a more pedestrian friendly place.

4. Increase the scale of design dimensions to accommodate large trucks, limited only to key intersections and turning locations. In such instances, additional design elements to protect and improve the pedestrian experience (e.g. channelized right turns) should be considered.

Credit: CH2M HILL

While channelized right turn lanes are not typically favored in areas of significant pedestrian presence, they can improve the crossing condition if and when large vehicles need to be accommodated.  A well-designed channelized right turn, as shown at right, can reduce vehicular speed through the right turn movement by 5 to 10 mph. Chapter 10 of the ITE publication Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities is a good resource for additional guidance.

Resources

PDFs:

• AASHTO Policy on Geometric Design of Highways and Streets
• Charlotte Curb Return Radii Design Guidelines
• Intersection Design
• ITE Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities (See Chapters 7 and 10)

Links:

• FHWA Flexibility in Highway Design, Chapter 4 Design Controls
• Massachusetts Design Guide, Chapter 3 Design Controls

Other References:

• AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities, Chapter 2 and 3
• AASHTO Policy on Geometric Design of Highways and Streets (Green Book), Chapter 2 and Chapter 7
• ITE Traffic Engineering Handbook