Multistate Safety Evaluation of Intersection Conflict Warning Systems

Intersection conflict warning systems (ICWSs) were evaluated under the FHWA Evaluation of Low-Cost Safety Improvements Pooled Fund Study. The ICWS strategy is intended to reduce the crash frequency by alerting drivers of conflicting vehicles on adjacent approaches at unsignalized intersections. The evaluation used a multistate database of geometric, traffic, and crash data for rural four-legged, two-way, stop-controlled intersections equipped with ICWSs in Minnesota, Missouri, and North Carolina. To account for potential selection bias and regression to the mean, an empirical Bayes before–after analysis was conducted by using safety performance functions (SPFs) for reference groups of similar intersections without ICWS installation. These SPFs also controlled for changes in traffic volumes over time and time trends in crash counts unrelated to the strategy. The aggregate results indicate statistically significant crash reductions at the 5% level for all crash types for two-lane-at-two-lane intersections and four-lane-at-two-lane intersections. For two-lane-at-two-lane intersections, the crash modification factors (CMFs) for total crashes, fatal and injury crashes, and right-angle crashes are 0.73, 0.70, and 0.80, respectively, and for four-lane-at-two-lane intersections, they are 0.83, 0.80, and 0.85, respectively. The benefit–cost (B:C) ratio estimated with conservative cost and service life assumptions is 27:1 for all two-lane-at-two-lane intersections and 10:1 for four-lane-at-two-lane intersections with post-mounted warning signs. The results suggest that the strategy, even with conservative assumptions on cost, service life, and the value of a statistical life, can be highly cost-effective. As this strategy is evolving, this study reflects installation practices to date.