The Federal Highway Administration (FHWA) organized a pooled fund study of 38 States to evaluate low-cost safety strategies as part of its strategic highway safety effort. The purpose of the FHWA Low-Cost Safety Improvements Pooled Fund Study is to evaluate the safety effectiveness of several low-cost safety strategies through scientifically rigorous crash-based studies. One of the strategies selected for evaluation for this study was the application of shoulder rumble strips (SRS) and centerline rumble strips (CLRS) in combination. This strategy is intended to reduce the frequency of crashes by alerting drivers that they are about to leave the travelled lane. While research has been published on the safety effectiveness of SRS or CLRS used in isolation, the effectiveness of the combined treatment has not been shown. 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-tothe-mean, an Empirical Bayes (EB) before-after analysis was conducted using reference groups of untreated two-lane rural roads with similar characteristics to the treated sites. A slightly different approach was required for the analysis of the treatment sites in Missouri, which is installing rumble strips on two-lane rural roads whenever a resurfacing project is undertaken. As a result, a suitable reference group with no rumble strips for this road type presently or in the near future did not exist. The analysis also controls for changes in traffic volumes over time and time trends in crash counts unrelated to the treatment. The combined results for all States indicate reductions in crashes for all crash types analyzed that are statistically significant at the 95-percent confidence level (i.e., 5-percent significance level). The crash type with the smallest crash modification factor (CMF) (i.e., the great 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 sideswipeopposite-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 (FI) were estimated. It is important to remember that all crash types considered exclude intersectionrelated and animal crashes. The disaggregate analysis sought to identify those conditions under which the treatment is most effective. Run-off-road, head-on, and sideswipe-opposite-direction crashes were the focus of this analysis because they are the focus of this treatment. The analysis found no clear trend between the CMF and values for posted speed, lane width, or shoulder width. Larger percentage crash reductions were found for run-off-road crashes at higher average annual daily traffic (AADT). For head-on+sideswipe-opposite-direction crashes, the trend is reversed with smaller percentage crash reductions at higher AADTs. For the expected crash frequency, larger percentage crash reductions were found for run-off-road crashes for higher crash frequencies. For head-on+sideswipe-opposite-direction crashes, the trend is reversed with smaller percentage crash reductions at higher crash rates. Because expected crashes increase with volume as seen in the Safety Performance Functions (SPF) developed, the trend of lower percentage crash reductions at higher crash rates for head-on+sideswipe-oppositedirection crashes would be expected given the results for AADT. Benefit-cost (B/C) ratios are estimated to range from 20.2 for a higher cost/higher service life assumption (based on Kentucky information) to 54.7 for a lower cost/lower service life assumption (based on information from Missouri). These results, which are based on conservative service life assumptions, suggest that the treatment, even in its most expensive variations, can be highly cost effective. (Author/publisher)
Samenvatting