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

What Is Hit?

The Michigan study found that the objects hit by drift-off vehicles included parked vehicles On file shoulder (4 percent), trees (13 percent), signposts and light poles (6 percent), guardrails and bridge rafts (19 percent), and cross-medians affecting an opposite-direction vehicle (1 percent).

The data are significant not for what was hit, but for what was not hit: 47 percent of the 1,887 drift-off vehicles hit no fixed object at all. The drivers of these vehicles, once they dropped a wheel off the paved shoulder, simply had trouble steering the vehicle to safety.

A few of these vehicles hit the backside of a ditch, but most simply came to harm trying to negotiate the side slope. On all of the roads in this section of the Michigan study, the side slopes are American Association of State Highway and Transportation Officials (AASHTO) standard (4-to-1) or better.

Additional analysis tells the story: Of the 1,805 vehicles in the study that drifted beyond the paved shoulder, 35 percent of the drivers overcorrected their steering. Forty-five percent of these same 1,805 vehicles rolled over during the crash. Simply dropping the wheel off the paved shoulder was enough to start the process for many of these vehicles.

In this study, 68 percent of the drivers who oversteered continued on to roll over.

Drift-off Crash Rate--A Moving Target

The researchers used 6 years of crashes on 762 kilometers (473 miles) of nonrumbled Michigan freeway to investigate the effect of traffic volume on drift-off crashes. Although the number of drift-off crashes showed little correlation with traffic volume, the rate of drift-off crashes is extremely sensitive. With increasing traffic volume, the rate of drift-off crashes decreases markedly. On nonrumbled road sections, a doubling in average daily traffic is likely to decrease the drift-off crash rate by 30 to 50 percent.

Most likely, two circumstances are driving the drift-off crash rate lower as traffic increases. First, higher traffic volumes require a higher level of alertness just for the routine task of driving. Second, a drowsy or distracted driver may indeed cause an accident on a high-volume road, but it may become a multivehicle crash that would not have been detected in the drift-off crash screening for the Michigan study.

Curves Linked to Crash Rate

A closer look revealed an unexpected piece of information about curves. When the researchers grouped the data according to mad curvature (percent of road in horizontal curvature), this created a set of trendlines showing a pattern. In each successive group of data, as the percent of road in curvature increases, so does the crash rate trendline. In other words, on the Michigan freeways, more horizontal curvature is associated with higher drift-off crash rate.

"It's Obvious that when drivers become drowsy, they are more prone to drift off the road on curves," says Culp. "The sharper the curve, the less time a driver has to correct the drift-off, and the crash rate goes up."