Safety Evaluation of Intersection Conflict Warning Systems

Chapter 9. Summary and Conclusions

The objective of this study was to undertake a rigorous before–after evaluation of the safety effectiveness of ICWS as measured by crash frequency. The study used data from three States, Minnesota, Missouri, and North Carolina, to examine the effects for the following specific crash types: total, fatal and injury, right-angle, rear-end, and nighttime crashes. Based on the combined results, the CMFs shown in table 26 are recommended for the various crash types.

Statistically significant results at the 95-percent confidence level are indicated in boldface.

The aggregate results indicated statistically significant crash reductions at the 95-percent confidence level for all crash types except nighttime crashes for two-lane at two-lane intersections. The results also indicated statistically significant crash reductions in all crash types except rear-end crashes for four-lane at two-lane intersections.

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 the focus of the disaggregate analysis. The disaggregate analysis of the results for two-lane at two-lane intersections indicated larger percentage crash reductions for sites with ICWSs installed on the major route, particularly for post-mounted ICWSs in advance of the intersection. An additional benefit may be provided by including the “WHEN FLASHING” message as part of the system. The disaggregate CMFs 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 disaggregate CMFs can be used in prioritizing installation sites, but again, interpretations should be made with caution.

The 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 intersections. With the 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 this is an evolving strategy, this study reflects installation practices to date. Future studies may show different results as installation practices change. In particular, the use of an overhead ICWS on the major route was limited to the 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 MUTCD provides guidance for the placement of warning signs so that they provide adequate PRT.⁽²⁾