Evaluation of Safety Performance of Passing Relief Lanes

This paper documents the evaluation of the safety performance of passing relief lanes. A passing relief lane is an intermittently spaced additional lane on a two-lane road that provides drivers the opportunity to pass without having to cross into opposing traffic. The operational benefits are well known, but there is limited substantive evidence on the safety of these lanes. The study was based on data from Michigan, specifically traffic volume and crash history data for seven sites at which passing lanes were implemented within the study period, and for 100 reference sites (without passing lanes) and 231 passing lanes sites that existed throughout the study period. The analysis involved an empirical Bayes before–after evaluation and a comparative, cross-sectional evaluation of safety performance of locations with and without passing lanes. The methodologies were also applied to adjacent nontreated sites 1 mi upstream and downstream of the passing relief lane segment to examine possible spillover effects. From the results, crash modification factors (CMFs) were established for passing lanes in Michigan. These CMFs, which indicated significant safety benefits from passing relief lanes, may be considered for use in locations in other jurisdictions. The results from the cross-sectional and before–after evaluations were generally consistent.