• September 20th, 2015

FHWA Publication No.: FHWA-HRT-15-064

The objective of this study was to perform a rigorous before-after evaluation of the safety effectiveness, as measured by crash frequency, of shoulder and centerline rumble strips applied in combination on two-lane rural roads.

The dual application of centerline rumble strips in combination with shoulder rumble strips is a recommended strategy in volumes 4 and 6 of the National Cooperative Highway Research Program (NCHRP) 500 Series Guidebooks.(1,2) These guidebooks describe shoulder rumble strips as 0.5-inch-deep, crosswise grooves in the road shoulder that are spaced about 7 inches apart and cut in groups of four or five. States have developed various designs and methods of installation, including rolling the rumble strips into hot asphalt or concrete as they are laid or milled in later. The rumble strips produce a vibrotactile—or auditory—warning in the form of a sudden rumbling sound or vibration to inattentive, drowsy, or sleeping drivers that encroach on the shoulder. Shoulder rumble strips are used extensively in the United States on all types of roadways.

Centerline rumble strips are similar to shoulder rumble strips but are placed on the center line, typically extending into the travel lane by 5 to 18 inches. They may be placed continuously or with periodic gaps. Shoulder and centerline rumble strips are compatible with other measures taken to reduce crashes (e.g., curve flattening) and may be included in existing construction plans with minimal extra cost.

A literature review revealed that while research into the safety performance of shoulder and centerline rumble strips that have been applied separately has been conducted, most notably for NCHRP Report 641, the combined application of shoulder and centerline rumble strips has been relatively rare, and evaluations to date have, as a result, been limited in scope.(3) The one U.S. study of relevance evaluated the safety impacts of applying centerline and shoulder rumble strips in combination using data for 80 mi of rural two-lane roads in Mississippi and applied the empirical Bayes (EB) before-after approach.(4) Target collisions were defined as the sum of head-on, sideswipe-opposite-direction, and single-vehicle run-off-road collisions. The results showed a 35-percent reduction in target collisions of all severities and a 39.6-percent reduction in fatal+injury target collisions. A Canadian study evaluated the safety impacts of applying centerline and shoulder rumble strips alone and in combination on two-lane rural and four-lane divided rural highways in British Columbia, Canada.(5) The results of this EB before-after study indicated that the combined application on two-lane roads indicated a reduction of 21.4 percent in off-road right, offroad left, and head-on collisions combined.

This study builds on these limited efforts using a multi-State database.