Safety Evaluation of Intersection Conflict Warning Systems

Executive Summary

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. The purpose of the FHWA Evaluation of Low-Cost Safety Improvements Pooled Fund Study is to evaluate the safety effectiveness of high-priority, low-cost safety strategies selected by member States through scientifically rigorous crash-based studies. One of the strategies selected by member States for evaluation was the application of 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, particularly those with one-way or two-way stop control. Few studies have explored the safety effectiveness of an ICWS; no studies have evaluated their effectiveness at four-legged intersections using a statistically rigorous methodology, such as the empirical Bayes (EB) before–after method.

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 (RTM), an EB before–after analysis was conducted using reference groups of similar four-legged, rural, two-way stop-controlled intersections without ICWS installation. Separate analyses were conducted for intersections with two lanes or four lanes on the major approaches. 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 reductions for all crash types analyzed (i.e., total, fatal and injury, right-angle, rear-end, and nighttime) for both two-lane at two-lane intersections and four-lane at two-lane intersections. The reductions were statistically significant at the 95‑percent confidence level for all crash types except nighttime crashes for two-lane at two-lane intersections. The reductions were statistically significant at the 95-percent confidence level for all crash types except rear-end crashes 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 0.733, 0.701, 0.803, and 0.425, respectively. Nighttime crashes had an estimated CMF of 0.898, which was not statistically significant at the 95‑percent confidence level. It is important to consider the sample size used to develop the CMF when interpreting the results because some of the CMFs were based on relatively small samples.

For four-lane at two-lane intersections, the statistically significant CMFs for total, fatal and injury, right-angle, and nighttime crashes were 0.827, 0.802, 0.850, and 0.612, respectively. Rear-end crashes had an estimated CMF of 0.973, which was not statistically significant at the 95‑percent confidence level.

The disaggregate analysis sought to identify those conditions under which the ICWS strategy was most effective. Because total, fatal and injury, and right-angle crashes were the focus of this strategy, these crash types were also the focus of the disaggregate analysis. Because installation category was the main factor for the disaggregate analysis, the categories developed by the North Carolina Department of Transportation (NCDOT) were expanded for use in this study.⁽¹⁾

Categories for further analysis included the following:

• Category 1—Overhead signs and flashers at the intersection on major; loop on minor.
  
  
• Category 2—Overhead signs and flashers at the intersection on minor; loop on major.
  
  
• Category 3a—Post-mounted signs and flashers in advance of the intersection on major; loop on minor.
  
  
• Category 3b—Post-mounted signs and flashers at the intersection on minor; loop on major.
  
  
• Category 4—Locations with a combination of category 1 through category 3.

The disaggregate analysis for two-lane at two-lane intersections indicated larger percentage crash reductions for sites with an ICWS installed on the major route, particularly for a post-mounted ICWS in advance of the intersection. Additional benefit may have been provided by including the “WHEN FLASHING” message as part of the system. The CMFs from the disaggregate analysis can be used in prioritizing installation sites, but interpretations should be made with caution. One should pay particular attention to the sample size used to develop the CMFs.

The disaggregate analysis for four-lane at two-lane intersections indicated larger percentage crash reductions for sites with intersection lighting and for sites with a higher expected average crash frequency in the before period. There was no substantive difference for sites with warning on the major route versus warning on the minor route. The CMFs from the disaggregate analysis can be used in prioritizing installation sites, but interpretations should again be made with caution.

The benefit-cost (B/C) ratio estimated with conservative cost and service life assumptions and, only considering the benefits for total crashes, was 27:1 for all two-lane at two-lane intersections and 10:1 for four-lane at two-lane intersections. The benefits were calculated from the significant reduction found for combined States for all two-lane at two-lane intersections and based on the statistically significant reduction found for four-lane at two-lane. With the U.S. Department of Transportation (USDOT)-recommended sensitivity analysis, these values could range from 16:1 to 39:1 for two‑lane at two-lane intersections and 6:1 to 14:1 for four-lane at two-lane intersections. These 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 ICWS is an evolving strategy, this study reflected installation practices to date. Future studies may show different results as installation practices change. In particular, the use of overhead ICWSs on the major route was limited to installations at the intersection (i.e., no advance warning), while post-mounted ICWSs on the major route were installed in advance of the intersection. Future research should compare these installation practices, considering placement of warning signs. Specifically, section 2C.05 of the Manual on Uniform Traffic Control Devices (MUTCD) provides guidance for the placement of warning signs so that they provide adequate perception-response time (PRT).⁽²⁾