Study Details

Study Title: Safety Evaluation of Offset Improvements for Left-Turn Lanes

Authors: Persaud et al.

Publication Date: JUN, 2009

Abstract: The Federal Highway Administration (FHWA) organized a pooled fund study of 26 States to evaluate low-cost safety strategies as part of its strategic highway safety effort. One of the strategies chosen to be evaluated for this study was offset improvements for left-turn lanes. This strategy is intended to reduce the frequency of crashes by providing better visibility for drivers that are turning left. The safety effectiveness of this strategy has not been thoroughly documented, and this study is an attempt to provide an evaluation through scientifically rigorous procedures. Geometric, traffic, and crash data were obtained for 92 installations in Nebraska, 13 in Florida, 12 in Wisconsin, and for a number of untreated reference sites in each State. To account for potential selection bias and regression-to-the-mean, an Empirical Bayes (EB) before-after analysis was conducted to determine the safety effectiveness of improving the offset for left-turn lanes. There was a large difference in observed effects among the three States, which may be explained, in part, by the wide variation in offset improvements. Florida and Nebraska employed pavement marking adjustments or minor construction to improve the offset. While the offset was improved at each site, most improvements did not result in a positive offset. Wisconsin reconfigured left-turn lanes through major construction projects, resulting in significant positive offsets. Results in Florida and Nebraska showed little or no effect on total crashes. Wisconsin showed significant reductions in all crash types investigated - total (34 percent), injury (36 percent), left-turn (38 percent), and rear-end (32 percent). A disaggregate analysis was conducted for Nebraska, the only State with enough installations to disaggregate the results. The analysis revealed that the percent reduction in crashes increased as the expected number of crashes increased. An economic analysis was conducted to identify the level of expected crashes that would yield a crash benefit to justify the construction cost. Based on this analysis, offset improvement through reconstruction is cost-effective at intersections with at least nine expected crashes per year and where left-turn lanes are justified by traffic volume warrants.

Study Citation: Persaud, B., C. Lyon, K. Eccles, N. Lefler, and F. Gross. "Safety Evaluation of Offset Improvements for Left-Turn Lanes." Report No. FHWA-HRT-09-035. Federal Highway Administration. Washington, DC. (June 2009)

Study Report: Download the Study Report Document


CMFs Associated With This Study

Category: Intersection geometry

Countermeasure: Improve left-turn lane offset to create positive offset

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.66233.83 StarsAllAllNot specifiedNot specified
0.64435.63 StarsAllK,A,B,CNot specifiedNot specified
0.62383 StarsLeft turnAllNot specifiedNot specified
0.68331.73 StarsRear endAllNot specifiedNot specified