Per the Caltrans Highway Design Manual Index 309.1(2), the roadside environment can and should be made as safe as practical. A clear recovery zone is an unobstructed, relatively flat (4:1 or flatter) or gently sloping area beyond the edge of the traveled way which affords the drivers of errant vehicles the opportunity to regain control.
The AASHTO Roadside Design Guide provides detailed design guidance for creating a forgiving roadside environment. Channels should be safe for vehicles accidentally leaving the traveled way. The figure below illustrates the preferred geometric cross section for ditches with gradual slope changes in which the front and back slopes are traversable (AASHTO 2002).
This figure (from HDS-4 Section 5.1) is applicable for rounded ditches with bottom widths of 8 ft (2.4 m) or more, and trapezoidal ditches with bottom widths equal to or greater than 4 ft (1.2 m).
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