Safety Evaluation of Intersection Conflict Warning Systems

The Federal Highway Administration (FHWA) organized a pooled fund study of 40 States to evaluate low-cost safety strategies as part of its strategic highway safety effort. One of the strategies selected for evaluation was intersection conflict warning systems (ICWSs). This strategy is intended to reduce the frequency of crashes by alerting drivers to conflicting vehicles on adjacent approaches at unsignalized intersections. Geometric, traffic, and crash data were obtained for four-legged, rural, two-way stop-controlled intersections with ICWS installations 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, using reference groups of similar four-legged, rural, two-way stop-controlled intersections without ICWS installation. The analysis also controlled for changes in traffic volumes over time and time trends in crash counts unrelated to the strategy. The combined results for all States indicated statistically significant crash reductions for most crash types for two-lane at two-lane intersections and for four-lane at two-lane intersections. For two-lane at two-lane intersections, the statistically significant crash modification factors (CMFs) for total, fatal and injury, right-angle, and rear-end crashes were 0.73, 0.70, 0.80, and 0.43, respectively. For four-lane at two-lane intersections, the statistically significant CMFs for total, fatal and injury, right-angle, and nighttime crashes were 0.83, 0.80, 0.85, and 0.61, respectively. The benefit-cost ratio estimated with conservative cost and service life assumptions was 27:1 for all two-lane at two-lane intersections and 10:1 for four-lane at two-lane intersections. The results suggest that the ICWS strategy, even with conservative assumptions on cost, service life, and the value of a statistical life, can be cost effective. Because this is an evolving strategy, the results of this study reflect installation practices to date.