Shoulder rumble strips (SRS) are continuous bands of either raised material formed on, or indentations grooved in, the shoulders of the road to alert drivers starting to drift off the road. SRS alert drivers to take corrective action before running off the roadway by the transmission of sound and vibration through the vehicle. SRS installation is now being performed on two-lane rural roadways and multi-lane divided highways. The safety effects of the use of SRS on these roadways are not as well documented as on higher functional class facilities. The objective of this research effort is to examine the effect on safety of SRS constructed on rural multi-lane divided highways. Highway Safety Information System (HSIS) data from Minnesota were used in this study.
The Minnesota Department of Transportation (MNDOT) provided specific data on SRS construction sites and SRS project data were compiled to perform a before-after safety analysis. HSIS information (e.g., highway design and accident type) was taken for twenty-three SRS sites and eight comparison sites. This study employed two before/after safety study approaches: 1) naive before/after study examining actual rates for the before and after construction periods, as well as the predicted after period rates while accounting for variations in traffic flow; 2) before/after with yoked comparisons involving one-to-one matching between study sites and reference sites. The results of the naive before after analysis showed that the average reduction in total crashes and injuries at the SRS sites was 16 percent and 17 percent respectively. The naive before after analysis results for single vehicle run off road (SVROR) crashes and injuries at the SRS sites was 10 percent and 22 percent respectively.
The yoked comparison before after analysis yielded the following results; for total crashes and injuries there was an 21 percent and 26 percent potential reduction respectively. The SVROR crash and injuries yielded a potential reduction of 22 percent and 51 percent respectively. Based on the naive before/after study, the reduction of crashes for rural multi-lane highways was found to be lower than the values in the literature for freeways; it is the authors opinions that a combination of analysis procedures utilized and functional class of facility contribute to this difference.