ANN ARBOR—Unlike alcohol, illicit drugs such as marijuana, cocaine and opiates are not a major factor in motor vehicle crashes, according to University of Michigan researchers.

In their study of more than 700 drivers treated for automobile crash injuries at hospital emergency rooms in Michigan, researchers at the U-M Transportation Research Institute (UMTRI) found that more than 15 percent of motorists had been drinking, while less than 8 percent tested positive for drugs. Another 6 percent had both been drinking and using drugs.

“Based on analyses of blood samples drawn within six hours of the crash, alcohol was found to be the major drug associated with injury,” says UMTRI director Patricia F. Waller. “Those patients testing positive for drugs, but not alcohol, had crashes that were very similar to those of drivers testing negative for both alcohol and drugs.

“These findings are not consistent with studies reporting that illicit drugs are a major factor in motor vehicle crashes.”

According to the study, drunk drivers are more likely to suffer fatal and serious injuries (55 percent), engage in hazardous actions leading to a crash (96 percent), and sustain more severe vehicular damage (65 percent) than those drivers who have not been drinking, regardless of illicit drug use.

Further, about two-thirds of the crashes of drunk drivers involve a single-vehicle collision occurring at night, with nearly half taking place on weekends. On the other hand, illicit drug use is not related to these conditions, and crashes of those testing positive for drugs are more similar to crashes of drivers not under the influence of either alcohol or drugs.

“These findings do not mean that illicit drugs do not affect driving performance,” Waller says. “The circumstances under which these drugs are used are probably very different from those involving alcohol. However, at least for the area covered by this study, alcohol remains the major drug of concern on the highway.”

Waller says that while police officers are adept at detecting alcohol use by drivers, it may be harder to determine if motorists are under the influence of illicit drugs.

“The fact that the crashes of drug-positive drivers are so similar to those of other drivers, with only the presence of alcohol making a difference, is probably a major reason for the difficulty seen for identifying drug use,” she says.

The study found that a third of the drivers in the sample had a history of alcohol abuse, while about 14 percent had a record of drug abuse. About half of each of these groups were alcohol- and drug-free at the time of their crash injuries.

“Perhaps the most interesting finding regarding alcohol and drug history is the large proportion of persons with evidence of alcohol or drug abuse or dependence who tested negative for both alcohol and drugs,” Waller says. “This provides encouragement for the possibility of combatting alcohol and drug use, particularly in combination with driving.”

Waller’s study appears in the current issue of the journal Accident Analysis and Prevention. Her colleagues on the study included U-M researchers Frederic C. Blow, Ronald F. Maio, Kathleen Singer, Elizabeth M. Hill and Nancy Schaefer.

U-M News and Information ServicesUniversity of Michigan