Safety Evaluation of the Combined Effect of Offset Left-Turn Lanes and Flashing-Yellow-Arrow Signals at Signalized Intersections on Multilane Divided Highways in Alabama Using the Empirical Bayes Method

The present study conducted an empirical Bayes (EB) before-after analysis to investigate the combined effects of the offset left-turn lanes and flashing-yellow-arrow (FYA) signals implemented at signalized intersections on multilane, divided highways in Alabama. A total of 35 signalized intersections were selected for the EB safety analysis. Among them, 30 intersections were classified as a reference group and five were classified as a treatment group. The reference group includes intersections which have not undergone any left-turn treatments from the period of 2010 to 2020, while the treatment group includes those improved with offset left-turn lanes and FYA signal implementation concurrently during years in that period. Safety performance functions were developed with data collected at reference-group intersections to predict crashes at such intersections under a no-treatment scenario. A study focus was then given to understanding the change in crash frequency before and after the combined treatments for the treatment-group intersections, using the EB method. Results show that the combined left-turn treatments (i.e., implementing offset left-turn lanes coupled with FYA signals) could reduce different types of crashes effectively. There was a substantial reduction of 27% in total crashes (crash modification factor [CMF] = 0.73), a 43% decrease in left-turn crashes (CMF = 0.57), and a 36% reduction in total injury crashes (CMF = 0.64) after the treatments. These findings were supported by their respective standard errors, which are 0.060, 0.101, and 0.106 for total, left-turn, and total injury crashes, respectively, and all the CMFs are statistically significant at 95% confidence intervals.