The AASHTO Green Book (2) provides a great deal of flexibility in intersection design, which is appropriate given its complexity and the large number of factors the designer should consider. Design guidance rather than prescribed minimum dimensions is discussed in the AASHTO Green Book (2).

The AASHTO Green Book (2) provides a great deal of flexibility in intersection design, which is appropriate given its complexity and the large number of factors the designer should consider. Design guidance rather than prescribed minimum dimensions is discussed in the AASHTO Green Book (2).

While a great amount of technical detail on the design dimensions is required for any given design vehicle, design vehicles for different cases or conditions are not specified or mandated in the AASHTO Green Book (2). Designers should use their own judgment in the selection of an appropriate design vehicle. Once a design vehicle is selected, the AASHTO Green Book (2) gives considerable guidance on radii and lane widths that are consistent with the characteristics of the design vehicle. (Note that federal and state legislation may require a route to be "accessible" for specified types of vehicles).

In terms of flexibility, designers and other stakeholders should be aware of the more stringent requirements governing intersections associated with the Manual on Uniform Traffic Control Devices (MUTCD). This document specifies practice regarding the design and placement of traffic-control devices, including traffic signals, stop and other regulatory signs, and warning signs. Many of the dimensions, treatments, etc., in the MUTCD are mandated.

From A Guide for Achieving Flexibility in Highway Design, 2004, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. AASHTO publications may be purchased from that organization's bookstore at 1-800-231-3475 or online at http://bookstore.transportation.org.

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