Safety Evaluation of Multiple Strategies at Stop-Controlled Intersections

Executive Summary

The Federal Highway Administration (FHWA) established the Development of Crash Modification Factors (DCMF) program in 2012 to address highway safety research needs for evaluating new and innovative safety strategies (improvements) by developing reliable quantitative estimates of their effectiveness in reducing crashes. The ultimate goal of the DCMF program is to save lives by identifying new safety strategies that effectively reduce crashes and promote those strategies for nationwide implementation by providing measures of their safety effectiveness and benefit–cost (B/C) ratios through research. State transportation departments and other transportation agencies need to have objective measures for safety effectiveness and B/C ratios before investing in broad applications of new strategies for safety improvements. Forty State transportation departments provided technical feedback on safety improvements to the DCMF program and implemented new safety improvements to facilitate evaluations. These States are members of the Evaluation of Low-Cost Safety Improvements Pooled Fund Study (ELCSI-PFS), which functions under the DCMF program.

This study evaluated multiple low-cost treatments at stop-controlled intersections in South Carolina. Improvements included basic signing and pavement markings. The purpose of this study was to quantify the safety effectiveness of these treatments. The treatments were installed as part of the South Carolina Department of Transportation (SCDOT) systemic intersection improvement program. The results provide evidence for others to consider as they make data-driven decisions about the type of treatments to implement.

Both urban and rural stop-controlled intersections on divided and undivided State-maintained roads (nonfreeways) were selected as locations for treatments. Preliminary study results have shown that, by making improvements such as those listed above, SCDOT was able to achieve a small but statistically significant crash reduction. While the expected crash savings per location were not as large as for some higher cost treatments (e.g., converting conventional intersections to roundabouts), the low cost of these treatments allows many more locations to be treated.

Geometric, traffic, and crash data were obtained at three- and four-legged, two- and four-lane major road, and urban and rural stop-controlled intersections in South Carolina. To account for potential selection bias and regression to the mean (RTM), an empirical Bayesian (EB) before–after analysis was conducted, using reference groups of untreated intersections with similar characteristics to the treated sites. The analysis also controls for changes in traffic volumes throughout time and time trends in crash counts unrelated to the treatments.

The aggregate results indicate reductions for all crash types analyzed (i.e., total, fatal and injury, rear-end, right-angle, and nighttime). The reductions are statistically significant at the 95-percent confidence level for all crash types. For all crash types combined, the crash modification factors (CMFs) are 0.917 for all severities and 0.899 for fatal and injury crashes. The CMFs for rear-end, right-angle, and nighttime crashes are 0.933, 0.941, and 0.853, respectively.

The disaggregate analysis identified those conditions under which the multiple low-cost treatments are most effective. Variables of interest included area type (urban or rural), number of legs (three or four), lane configuration of the mainline and the cross street, traffic volumes, and expected crashes without treatment. The disaggregate analysis indicated larger crash reductions of all types for rural areas, four-legged intersections, and intersections with two-lane major roads. For total entering volume and expected crashes before treatment, the disaggregate analysis indicated the strategy is more effective on average for intersections with lower traffic volumes and fewer expected crashes per year.

Assuming a 3-year service life, conservative costs, and the benefits for total crashes, the B/C ratio is 12.4 to 1. With the U.S. Department of Transportation (USDOT) recommended sensitivity analysis, these values could range from 7.1 to 1 up to 17.5 to 1. Assuming a 7-year service life, and the same conservative costs and benefits for total crashes, the B/C ratio is 25.5 to 1. With the USDOT recommended sensitivity analysis, these values could range from 14.5 to 1 up to 35.9 to 1. These results suggest that the multiple low-cost treatments, even with conservative assumptions on cost, service life, and the value of a statistical life, can be cost-effective in reducing crashes at stop-controlled intersections.