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.

Design Vehicle for Small Town Downtowns:

Several characteristics influence design vehicle selection for these place types:

• Large truck traffic is not uncommon, but most of it is "through traffic"; and
• Significant pedestrian presence is likely.

Select the smallest practical design vehicle. Small town downtowns generally should not be designed to accommodate large vehicles. Large trucks using a major thoroughfare to pass through do not require significant changes in the design of thoroughfares or intersections. 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 pedestrian activity;
• The smaller radii also minimize the speed at which vehicles can turn around corners, improving pedestrian safety; and
• Minimizes lane width requirements, which reduces the right-of-way required for vehicular traffic, leaving more space for pedestrian activity, landscaping, bicycles and other roadside improvements. Narrower lanes also result in a shorter crossing distance for pedestrians at intersection and mid-block crosswalks.

Considerations for larger trucks:

An exception to the guidance above is access for large trucks to fuel and supplies, or for delivery of goods to local businesses. These vehicles should be considered in the design vehicle guidance for key intersections and access points. There may be instances of a local factory or other large truck generator in close proximity to a small town downtown.  In such cases, the following options should be considered:

Credit: CH2M HILL

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. 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.

Source: 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 or suggestions.  Raising the crosswalk at channelized right turn locations with high pedestrian presence is recommended.  Doing so provides two additional benefits: 1) calming effect on vehicular travel; and 2) minimal grade changes for wheelchair pedestrians to navigate.

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
• ITE Traffic Engineering Handbook