Design Vehicle

Article

Design vehicle is defined by ITE as the vehicle that must regularly be accommodated on a thoroughfare without encroachment into other travel lanes. The design vehicle’s dimensions and movements can play a large role in the physical characteristics of a thoroughfare, 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 thoroughfare dimensions, particularly at intersections.

There are a number of tradeoffs inherent in design vehicle selection. The design vehicle selected for a given thoroughfare should represent the largest vehicle that regularly or frequently, not occasionally, uses it. Selecting too large a design vehicle for a thoroughfare or thoroughfare 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 Downtown Main Streets:

Several characteristics influence design vehicle selection for these place types:

• Large truck traffic is uncommon
• Transit vehicles are often present
• There is a significant pedestrian presence

Select the smallest practical design vehicle. Downtown main streets, small town downtowns, mixed-use districts, and residential areas should not be designed to accommodate large vehicles, generally speaking. These types of areas are typically comprised of numerous small parcels that rely on pedestrian traffic for business and access. 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:

• Minimal intersection and driveway “footprints.” By minimizing the turning radii requirements at corners, there is more space for adjacent land uses, including transit and pedestrian facilities;
• The smaller radii also minimize the speed at which vehicles can turn around corners, improving safety for all modes, including pedestrians and bicycles; and
• Minimal lane-width requirements reduce the right-of-way required for vehicular traffic, allowing more space for transit, pedestrian activity, landscaping, and various roadside improvements. Narrower lanes also result in a shorter crossing distances for pedestrians at intersections and mid-block crosswalks.

Credit: CH2M HILL

Plan and design for bus access. Because buses and light rail lines are common along downtown main streets, it is important to coordinate with regional transit agencies to determine the appropriate dimensions and considerations if a transit vehicle 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.

Strategically design for truck access. Large trucks are a component of commerce and shipping and though they would ideally be rerouted to mobility-oriented arterials, there may still be a need to accommodate large trucks delivering goods and materials to businesses along downtown main streets. The following design criteria can improve access for large trucks in downtown areas.

• 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.

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  Credit: CH2M HILL

  If large trucks must be accommodated, strategically increase the scale of key intersections and turning locations, rather than facilities throughout the corridor. In such instances, additional design elements to protect and improve the pedestrian experience (e.g. channelized right turns) should be considered.

• While channelized right turn lanes are not typically favored in areas of significant pedestrian presence, they can improve the crossing condition when large vehicles need to be accommodated.

  A well-designed channelized right turn 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 and suggestions.

Resources

PDFs:

• AASHTO Policy on Geometric Design of Highways and Streets
• Charlotte Curb Return Radii Design Guidelines
• Intersection Design (from Mass Highway)
• 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