Designers need to understand the nature and proper use of the Roadside Design Guide (3). While clear zone dimensions are provided in this guide, they should not be viewed as either absolute or precise. It is expected that the establishment of roadside design criteria and the design of the roadside is a site- or project-specific task for the designer.

Designers need to understand the nature and proper use of the Roadside Design Guide (3). While clear zone dimensions are provided in this guide, they should not be viewed as either absolute or precise. It is expected that the establishment of roadside design criteria and the design of the roadside is a site- or project-specific task for the designer. Also, the Roadside Design Guide (3) suggests that more than one solution may be evident or appropriate for a given set of conditions. Indeed, in presenting the core technical material on clear zone dimensions and design, it is noted that, "The designer must keep in mind site-specific conditions, design speeds, rural versus urban locations, and practicality. The distances obtained from Figure 3-1 and Table 3-1 [in the Roadside Design Guide (3)] should suggest only the approximate center of a range to be considered and not a precise distance to be held as absolute." The proper application of the guidelines in the Roadside Design Guide (3) should, in the long run, reduce the number and severity of roadside crashes without unreasonable costs or impacts to the surrounding environment.

Designers and the public should understand that, once a vehicle leaves the road, a crash or potentially serious encounter with the roadside may occur, regardless of the clear-zone width established by the designer. The selected clear-zone width is a compromise, based on engineering judgment, between what can practically be built and the degree of protection afforded the motorist. Limitations in available right-of-way, the location, frequency, and nature of roadside objects, or the presence of valued resources such as wetlands, or the need to provide for pedestrian or other activities may practically limit the clear-zone width. For both new and existing facilities, the selection and design of appropriate clear-zone dimensions should be the responsibility of a knowledgeable, professional design engineer trained and experienced in roadside design issues.

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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