The concept applied to lessening the severity of run-off-the-road accidents has been named "the forgiving roadside." Obviously, the least hazardous roadside environment would contain no objects that an errant vehicle could impact, but would allow the driver to regain control and either stop or return to the travel lane without injury or damage. The American Association of State Highway and Transportation Officials (AASHTO) introduced the concept of a roadside "clear zone" that was relatively flat and free of obstructions. It has become generally accepted that the width of the clear zone should depend on speed, and should be wider at those locations (like the outside of horizontal curves) where vehicles are more likely to leave the traveled portion of the roadway. Although the idea of a clear zone originated in the U.S. and much of the early research was done there, over the past 25 years the concept has been accepted as an important cross section design consideration in other countries. Current examples of clear zone criteria from several countries were gathered through an international survey and are presented in this paper. Essentially, there are four general categories of roadside design elements addressed in AASHTO's "Roadside Design Guide" (1988). These are (1) roadside topography or slopes, (2) drainage features, (3) highway appurtenances such as sign and light supports, and (4) traffic barriers. For each of these features, this paper reviews the historical development, describes the current situation, highlights existing shortcomings, and identifies where efforts might be directed in the near future.
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