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The Roadside: Flexibility in the AASHTO Guidelines

American Association of State Highway and Transportation Officials (AASHTO)

The AASHTO Green Book is intended to be flexible with respect to roadside design treatments. It refers to the Roadside Design Guide for use in general guidance.

Within the AASHTO Green Book, recommendations are made regarding clear-zone dimensions for different highway types in rural or urban areas. In rural areas, the recommended minimum is 10 ft (3 m); in urban areas the minimum value is 1.5 ft (0.5 m). A reading of the AASHTO Green Book discussion, however, reveals that such guidance is not prescriptive or mandatory, but advisory. Indeed, the urban value of 1.5 ft (0.5 m) is associated with operational needs (car door conflicts with roadside objects) and not strictly safety issues (i.e., impacts to objects beyond the pavement).

Designers and their customers – the public, local elected officials, and others – should acknowledge that it is not possible to design a totally risk-free roadside, and thus it is not reasonable to expect a totally risk-free roadside. Once a vehicle leaves the road, adverse consequences likely will result. Achieving the safest possible roadside on any given project will require the best efforts of the designer, working with the alignment, land use, and other constraints. In addition, it will take the best efforts of maintenance and permitting operations to keep the roadside clear of obstacles.

Designers working in the context-sensitive environment will be successful if they understand the content and intent of the Roadside Design Guide and the AASHTO Green Book and if they follow these general guidelines:

• Avoid setting an artificial design speed (i.e., unrealistically low).
• Apply a consistent roadside treatment approach for any project.
• Avoid the establishment of an arbitrary clear-zone width.
• Encourage the removal or relocation of signs, utility poles, and other fixed objects to improve both safety and aesthetics.
• Encourage safe landscaping, paying special attention to keep trees and tall bushes out of sight triangles at intersections, beyond the limits of the desired clear zone, and beyond the clear zone in medians on higher-speed highways and streets.

Designers should assume that vehicles would travel at the design speed or known operating speeds, rather than at the posted speed limit. If lower speeds than currently exist are desired, designers may opt for traffic-calming solutions intended to reduce speeds. As noted in earlier sections of this chapter, special attention to the roadside is appropriate where a design exception is required for other geometric features such as cross section or alignment.

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 https://bookstore.transportation.org.