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

Single-vehicle run-off-road crashes represent a large share of the Nation's crashes. The category is large ill part because it covers such at broad range of roadway departures. Included are intentional departures to avoid other vehicles or objects; involuntary departures due to the blowouts, ice, hydroplaning, or trailer sway; and driver errors such as distractions or drowsiness.

Within this category is a much smaller, more lethal crash type that is responsible for a disproportionate share of the severe run-off-road crashes. That lethal subcategory is the drift-off-road crash, defined as drivers who drift off the road due to drowsiness, inattention, or distraction. In the broader world of run-off crashes, these crashes are three-to-five times more severe than other run-off-road crashes, and they are highly correctible.

"Here in Michigan, we're focusing on reducing serious crashes, and countermeasures targeting the drift-off crashes are of prime importance in this effort," says James D. Culp, traffic and safety engineer, Michigan Department of Transportation (DOT). hi fact, a recent study in Michigan indicates that a 40 to 80 percent reduction in drift-off crashes is possible with proper design and installation of rumble strips.

A Close Look at Drift-Offs

Michigan researchers analyzed 1,887 reported drift-off crashes to examine the nature of this type of freeway incident. In compiling the database, they only reviewed crashes during rainy and dry weather and excluded crashes on icy or snowy roads. The study produced a number of findings regarding drift-off crashes.

First, in Michigan, time wide range of crashes that are classified as run-off-road produce a crash data set of low to moderate severity. For the years 1996-2001, on 875 kilometers (544 miles) of freeway, only 3.4 percent of all wet road run-off crashes resulted in a severe outcome (fatal or incapacitating injury,). Traffic fiction run-off crashes--where the driver was changing lanes, passing, or avoiding another vehicle--resulted in severe outcomes 6.1 percent of the time. The corresponding severity percentage for winter weather run-off crashes (snowy, icy, slushy road) is 2.9 percent; for vehicle defect runoff crashes, 6.0 percent.

Drift-off crashes are extremely severe, in comparison to these other run-off crashes. For the same Michigan roadways, where rumble strips were not present on the shoulders, 17 percent of drift-off crashes included at least one fatal or incapacitating injury. Even where shoulder rumble strips are present, 12 percent of drift-off crashes result in fatal or incapacitating injury.

The only crash groups that can get close to the severity of freeway drift-offs (12 to 17 percent severe outcome) are the behavioral groups (risky driving) usually addressed by nonengineering safety agencies. Even these crash groups, many of which are the subject of continuing national and local safety efforts, fall short whelm compared on the severity scale. In 1999 Michigan data, for instance, approximately 14 percent of crashes involving nonseatbelt usage resulted in death or incapacitating injury; alcohol 13 percent; red light running 7 percent; and speeding 6 percent.

Meet the Drift-Off Driver

In a majority of the Michigan drift-off crashes, die driver stated to the officer that he or she was distracted, drowsy, "must have been asleep," "looking at a map," or "can't remember what happened." These statements enabled the Corresponding crash reports to be classified specifically as either "drowsy" or "distracted."

Of these drift-off crashes with known causes, 82 percent of the drifting drivers were drowsy or asleep. Driver distraction, though topical in the safety community, is only a small percentage of the freeway drift-off crash problem.

Because of the national interest in drivers using cell phones, researchers further reviewed the 55 distracted crashes that occurred during 2000 and 2001 to identify the cause of the distraction. Cell phone usage in this study accounted for only 6 of these crashes (11 percent), most of which involved physical interaction with the phone, not the distraction of conversation.

The most common distraction in this group of crashes was drivers looking for, handling, or reaching for something, such as a CD, tape, or radio (9 crashes); food and drink (8); children in the back seat (3); pets (3); cigarettes or lighter (2).

In the overall study, 66 percent of all drift vehicle drivers were male, which is consistent with the overall Michigan crash data: 58 percent of all crash drivers and 73 percent of all fatal crash drivers are male.

An age profile of drift-off drivers indicates a trend similar to all Michigan crashes: high crash involvement of younger drivers, with declines in crash involvement as age progresses. Researchers found a slight shift toward the younger driver and noted no particular difference in the percentage of elderly drivers involved ha drift-off-road crashes versus all crashes.

About one-fifth of the crashes in the Michigan drift-off database involved a driver under the influence of alcohol or drugs at the time of the crash. The driver action in many of these crashes was slightly erratic or excessively slow, due often to drowsiness. In this study, crashes related to alcohol had comparable severity levels to those not related to alcohol.