The Intersection Conflict Warning Systems (ICWSs) strategy involves installing vehicle detectors, warning signs such as flashing beacons and variable message signs on the major and/or minor approaches of unsignalized intersections to detect and alert motorists about conflicting vehicles on adjacent approaches. This study evaluates ICWSs at four-legged rural two lane stop-controlled intersections. The objective was to estimate the safety effectiveness of this strategy as measured by crash frequency. Geometric, traffic, and crash data of this type of intersections with ICWS installations in Minnesota (MN) (13 intersection installations), Missouri (MO) (14 intersection installations), and North Carolina (NC) (66 intersection installations) were used to conduct the evaluation of their safety effectiveness. The categories of ICWS installations considered for this study included; i) Overhead signs and flashers at the intersection on the major; loop detector on the minor approach, ii) Overhead signs and flashers at the intersection on the minor approach; loop detector on the major approach, iii) Post-mounted signs and flashers in advance of the intersection on the major; loop detector on the minor approach and post-mounted signs and flashers at the intersection on the minor approach; loop detector on the major approach, iv)Locations with a combination of category (i) through category (iii).
In order 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. Each State in this study (MN, MO, and NC) identified approximately 30 reference sites for four-legged intersections with two lanes on the major route and 30 reference sites for four-legged intersections with four lanes on the major route. Target crash types included total crashes (all types and severities combined), injury crashes, right-angle crashes, rear-end crashes and nighttime crashes. A crash modification factor (CMF) was used to estimate the change in the expected number of crashes after implementing ICWS.
The Benefit-to-Cost ratio estimated with conservative cost and service life assumptions and by only considering the benefits for the total number of crashes. The statistically significant reduction in the total number of crashes for all three states was used as the benefit. Cost information is based on the average cost estimate for each installation type (e.g., overhead signs on both approaches) by major route number of approach lanes provided by NCDOT, MoDOT and MnDOT.
- The results showed 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 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.
- For two-lane at two-lane intersections, the statistically significant 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.
- Sensitivity analyses led to a range of 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 for benefit-cost ratios.
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 in reducing total crashes at four-legged intersections with stop-control on the minor approaches. 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 estimate the CMFs.