Safety Evaluation of Centerline Plus Shoulder Rumble Strips

The combined application of centerline and shoulder rumble strips was selected for evaluation under the Federal Highway Administration (FHWA) Evaluation of Low-Cost Safety Improvements Pooled Fund Study. This strategy is intended to reduce the frequency of crashes by alerting drivers that they are about to leave the travelled lane. To date, this combination treatment has not been rigorously evaluated with a multi-state database. Geometric, traffic, and crash data were obtained at treated two-lane rural road locations in Kentucky, Missouri, and Pennsylvania. To account for potential selection bias and regression-to-the-mean, an Empirical Bayes (EB) before-after analysis was conducted, utilizing reference groups of untreated two-lane rural roads with similar characteristics to the treated sites. The analysis also controlled for changes in traffic volumes and time trends in crash counts unrelated to the treatment. The combined results for all states indicate statistically significant crash reductions for all crash types analyzed. The crash type with the smallest crash modification factor (CMF) (i.e., the greatest crash reduction) is head-on with a CMF of 0.632. Run-off-road and sideswipe-opposite-direction crashes have estimated CMFs of 0.742 and 0.767, respectively. For run-off-road, head-on and sideswipe-opposite direction crashes combined (i.e., lane departure crashes), the estimated CMF is 0.733. For all crash types combined, CMFs of 0.800 for all severities and 0.771 for fatal+injury were estimated. Benefit-cost (B/C) ratios were estimated to range from 28.2 to 67.7, depending on the treatment cost and service life assumption, which varied by state.