NCHRP Research Report 1097 presents guidelines to determine a recommended clear recovery distance for a given set of roadway and roadside characteristics. Due to the limitations inherent in using crash data for this purpose, an innovative methodology was used that combined encroachment simulations, crash data, statistical modeling, and risk analysis. The guidelines include a risk-based tool that correlates recommended clear zone distance with potential for severe injuries or fatalities to motorists. The guidelines should be of interest to road design and safety professionals seeking to specify the safest and most practical clear zone recovery areas for various roadway designs.
The clear zone concept for roadside design emerged in the mid-1960s as a single distance for lateral clearance that reduced the likelihood of an errant vehicle striking a roadside obstacle. Subsequent recovery area guidelines that developed over the next two decades provided a variable distance expressed in terms of traffic volume, design speed, side slope, and other roadway and roadside factors. However, these values are based on studies from the 1950s through 1980s that used relatively limited data and extrapolated numbers. User agencies recognized a need for updated guidelines to aid designers in better understanding the risk associated with roadside encroachments while recognizing and working within the associated design constraints.
Under NCHRP Project 17-11(03), “Development of Clear Recovery Area Guidelines,” Texas A&M University Transportation Institute was asked to develop guidelines for roadside clear zones that are expressed in terms of key roadway and roadside design parameters. The research approach combined vehicle dynamics computer simulation results with crash data analyses. The vehicle dynamics simulations permitted consideration of a wide range of encroachment and design variables. Specialized crash datasets with reconstructed crashes were used to develop marginal probabilities for the encroachment variables, which were applied as weight factors to the simulation results. Statistical models were developed from the weighted simulation results. The statistical models were incorporated into an encroachment probability-based risk analysis tool. The analysis tool estimated the probability of a severe injury or fatal crash for a prescribed roadway and roadside configuration for a given lateral offset (i.e., clear zone distance) and a selected fixed object spacing at the clear zone edge. Clear zone guidelines were developed using a relative risk approach whereby the recommended clear zone distance has a risk of a severe injury or fatal crash that is less than or equal to that of a roadside guardrail. The guidelines were developed using equations and a chart-based format expressed in terms of design variables found to have the most significance on the clear zone risk, and hence are suitable for possible incorporation into the AASHTO Roadside Design Guide (RDG).
NCHRP Research Report 1097 is available on the TRB website at trb.org by searching for NCHRP Research Report 1097.
