Evaluating the Effectiveness of Continuous Shoulder Rumble Strips in Reducing "Ran-off-Roadway" Single-Vehicle Crashes

The research efforts presented in this report, prepared for the Nevada Department of Transportation, summarizes the evaluation of the effectiveness of continuous shoulder rumble strip treatment to reduce single-vehicle ran-off-roadway crashes in Nevada. The roadways studied included Interstate freeways, US routes, and state routes (SR) totaling 306 individual segments for a total of 1,303 centerline mile of roadways. Data for the period from 1995 to 2003 were used for the analyses. Key data considered in the analyses included the locations and dates of installation of continuous shoulder rumble strips, crash data, posted speed limits, and average daily traffic volumes. Crash records in Nevada for this period include over 33,000 ran-off-roadway single-vehicle crashes; of those 772 were fatal and 11,976 involved injuries. The crash data was determined for periods and the installation of the continuous shoulder rumble strips. Analyses of the data showed that overall the treatment has been effective in reducing the number of crashes and the corresponding crash rates. Linear regression models were developed to relate the crash rates during the before period to those during the after period. These models indicated that for any given crash rate, the predicted after crash rate was lower. The models suggested a reduction in the crash rates with higher values of average daily traffic, wider shoulders, and higher speeds. Roadways with posted speed limits greater than 65 miles/hour (105 kilometers/hour) showed significant improvements the installation of rumble strips based on crash rates and crash density. An analysis, based on Ezra Hauer's method for evaluation of safety, indicated that the installation of continuous shoulder rumble strips resulted in a significant reduction in the expected number of crashes on a vast majority of the roadways studied. Regression models developed indicated that the installations of rumble strips typically resulted in a reduction in the crash frequency, rate, or density. A comparison of the mean crash rates for roadways treated with rumble strips using Cox's method of comparing Poisson means further validated the finding that treatments were effective in reducing the crash rates on all roadways with the exception of one. In summary, the installation of continuous shoulder rumble strips on roadways in Nevada has resulted in improved safety for single-vehicle ran-off-roadway crashes. The outcomes of this research will assist transportation safety managers in Nevada and nationwide to better understand the effectiveness of continuous shoulder rumble strips in reducing ran-off-roadway single-vehicle crashes and identifying opportunities for applications of these rumble strips.