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

Trucks and buses are worth noting, first, because they are generally driven by professional drivers and, second, because they have large tires susceptible to vibration by milled rumble strips, but not by the other rumble designs used in Michigan. The Michigan study defined trucks as vehicles with a Gross Vehicle Weight Rating (GVWR) over 4,540 kilograms (10,000 pounds). The crashes reviewed included only one bus--a school bus. Although truck traffic accounted fur 11 percent of total vehicle-miles driven in this study, only 4.4 percent of the drift-off vehicles were trucks. This finding supports the common assertion that truck drivers are indeed more alert and drive more professionally than the average driver.

Time of Day and Day of Week

Throughout the entire 24-hour day, the number of drift-off crashes remains fairly level. A slight peak occurs in the early morning hours and again in the early "afternoon. Both of these time periods correspond to the cycle of sleepiness-alertness set the human body's circadian rhythm.

If the numbers were adjusted to reflect thalami large majority of travel is during daytime, however, the rate of drift-off crashes would be seen to be "sky-high" for the early morning hours and very low for daytime travel. The interpretation can be expressed this way: If an agency manages a section of freeway, expect a drift-off crash at any time. If a son or daughter is driving back to college, 1:00 a.m. is a very dangerous time to make that trip.

Thirty-eight percent of the crashes in the Michigan study occurred on Saturdays and Sundays--about one-third-higher than would be expected if the crashes were to occur randomly throughout the week. By comparison, only 25 percent of all Michigan freeway mainline (non-drift-off) crashes occur on a Saturday Or Sunday.

"Drivers making longer trips, such as those occurring on weekends, are more susceptible to drowsiness and drift-off crashes," says Culp.

What Can Happen to a Drift-Off Vehicle?

The researchers analyzed the Michigan crash reports to determine the side of the road from which the vehicles first exited the roadway. They defined "exiting the roadway" as touching a wheel beyond the paved shoulder, except where a drift vehicle actually Contacted a parked vehicle while still on the shoulder.

Nearly half of all drift-off vehicles exited in each direction (53 percent right, 47 percent left)--defying the common perception that fills type of incident is primarily a right-side crash. Even more illuminating were the 289 crashes that resulted in death or incapacitating injury: 53 percent of those drift vehicles exited to the left.

In Michigan, most of the paved right shoulders are 2.7-3 meters (9-10 feet) wide, and most of the paved left shoulders are 1.2 meters (4 feet) wide. Also, most Michigan freeways are crowned at or near the middle of the pavement. Because the crown is the high point in a cross-section of pavement, an "unsteered" (or drifting) tire will flow in a direction away from the crown.

That crowning may partially explain the equality in drift direction. However, the implication for road agencies remains the same: When applying a rumble strip countermeasure, treating the left shoulder is ms important as treating the right.