The clear zone concept for roadside design emerged in the mid-1960s, when the idea of a single lateral distance beyond which any potential roadside obstacle did not require removal or protection was introduced. However, acceptance of a single distance for lateral clearance has diminished over time. The Guide for Selecting, Locating and Designing Traffic Barriers (1977) and AASHTO's 1989 Roadside Design Guide provide guidelines for roadside recovery areas based on traffic volume, design speed, side slope, and other roadside conditions. Although these guidelines provide a more realistic approach than the application of a single distance, there are major concerns because the values are based on old studies that used relatively limited data and extrapolated numbers. Experience has also indicated that the recovery areas provided along highways is usually not completely clear of all objects and often has side slopes greater than desired. Further, transportation agencies frequently face difficulties in providing desirable recovery areas because of right-of-way constraints or construction costs. Consequently, current practice is to provide an area that provides a reasonable opportunity for a driver to regain a measure of control or to slow an errant vehicle. Updated guidelines are needed to aid designers in determining safe and cost-effective recovery areas, while recognizing the associated constraints.
The objective of this study is to develop guidelines for roadside clear zones that can be incorporated into the AASHTO Roadside Design Guide. This is the final stage of work begun in 17-11 and continued in 17-11(02).
STATUS: Research is in progress.