An important consideration in defining the appropriate cross section for a particular highway facility is the width of the clear zone. As defined in Chapter IV of the AASHTO Green Book, the clear zone is “…the unobstructed, relatively flat area provided beyond the edge of the traveled way for the recovery of errant vehicles.” The width of the clear zone is influenced by several factors, the most important of which are traffic volume, design speed of the highway, and slope of the embankments. The AASHTO Roadside Design Guide is a primary reference for determining clear zone widths for freeways, rural arterials, and high-speed rural collectors based on these factors. For low-speed rural collectors and rural local roads, the AASHTO Green Book suggests providing a minimum clear zone width of 3.0 m (10 ft). For urban arterials, collectors, and local streets with curbs, the space available for clear zones is typically restricted.
Source: Flexibility in Highway Design p. 81
Published: 1997
