Rumbling toward safety: Michigan study finds that the most severe run-off crash is the drift-off and that rumble strip design and placement significantly reduce these crashes - Cover Story

Public Roads, Sept-Oct, 2003 by David A. Morena

This explanation would be consistent with the concept of microsleeps, in which the brain stops processing information, even though the eyes may still be open. According to the Federal Motor Carrier Safety Administration, during a "microsleep" lasting 4 or 5 seconds, a car can travel 91.5 meters (100 yards), which is plenty of time to cause a serious crash.

Rumble Strip Comparisons

Three types of rumble strips are used on Michigan freeways: two older designs (rolled-in and concrete intermittent rumbles) and the current milled-in rumble design. The researchers matched the 1,887 Michigan crash reports to road sections of known average daily traffic and known rumble strip design, including sections with no rumbles. The drift-off crash data review Corresponded to 32 billion vehicle-miles of travel.

The advantage of the milled rumble strip over the earlier designs is that the milled cross-section is designed to allow vehicle tires to partially drop into them. This effect provides a vibration to the vehicle that translates up to the steering wheal. Whereas the roiled and concrete intermittent designs can provide some outside noise to alert a drifting driver, the milled design produces a louder noise and adds a vehicle vibration that most certainly increases the potential for alerting the drowsy or distracted driver.

The Michigan crash data bear out this logical progression--that more noise and more vehicle vibration lead to increased effectiveness. Milled rumble strips in Michigan reduced drift-off crashes by 40 percent, through the entire range of traffic volumes studied. The researchers calculated that, in the Michigan data, the two older designs--rolled-in and concrete intermittent rumbles--were approximately 20 percent effective in reducing drift-off crashes.

Although Michigan attributes a 40 percent reduction in drift-off crashes to time milled rumble strips, engineers with the Pennsylvania Turnpike Commission, New York State DOT, and New York State Thruway Authority reported crash reductions of 60 to 80 percent after installation of milled-in rumble strips.

What accounts for the comparatively lower reduction in drift-off crashes in Michigan? Possibly, the lateral location of the rumble strips makes a difference. The Michigan DOT installed its rumbles at 300-millimeter (12-inch) and 600-millimeter (24-inch) offset to the pavement edge line, whereas the Pennsylvania Turnpike, for instance, installed its rumbles at 100-millimeter (4-inch) offset. New York State DOT installs its freeway rumbles at 250 millimeter (10-inch) and 100 millimeter (4-inch) offsets. This suggests that the closer the rumble is to the edge line, the more effective it might be in reducing crashes.

If Rumble Strips Were King

"As an engineering agency, we are naturally somewhat conservative and therefore cautious about new ideas," says John Friend, director of the Bureau of Highways, Michigan DOT, "but several of our staff stepped forward and said, 'We've got a problem, and if rumble strips calm help, why not try them?'"