Two new studies shed light on driving dangers

The nation's decades-long campaign to combat drunk driving continues to make our roads safer, but use of marijuana and prescription drugs is increasingly prominent on the highways, creating new safety questions, according to a pair of ground-breaking studies released by the Department of Transportation's National Highway Traffic Safety Administration.

One study, the latest version of NHTSA's Roadside Survey of Alcohol and Drug Use by Drivers, found that the number of drivers with alcohol in their system has declined by nearly one-third since 2007, and by more than three-quarters since the first Roadside Survey in 1973. But that same survey found a large increase in the number of drivers using marijuana or other illegal drugs. In the 2014 survey, nearly one in four drivers tested positive for at least one drug that could affect safety.

The latest edition of the survey shows that the prevalence of alcohol use by drivers continues to drop. About 8% of drivers during weekend nighttime hours were found to have alcohol in their system, and just over 1% were found with 0.08% or higher breath alcohol content-the legal limit in every state. This is down by about 30% from the previous survey in 2007 and down 80% from the first survey in 1973.

But even as drinking and driving continues to fall, use of illegal drugs or medicines that can affect road safety is climbing. The number of weekend nighttime drivers with evidence of drugs in their system climbed from 16.3% in 2007 to 20% in 2014. The number of drivers with marijuana in their system grew by nearly 50%.

A second survey, the largest of its kind ever conducted, assessed whether marijuana use by drivers is associated with greater risk of crashes. The survey found that marijuana users are more likely to be involved in accidents, but that the increased risk may be due in part because marijuana users are more likely to be in groups at higher risk of crashes. In particular, marijuana users are more likely to be young men-a group already at high risk.

This was the most precisely controlled study of its kind yet conducted, but it measured the risk associated with marijuana at the levels found among drivers in a large community. Other studies using driving simulators and test tracks have found that marijuana at sufficient dosage levels will affect driver risk.

Jeff Michael, NHTSA's associate administrator for research and development, emphasized that, "Drivers should never get behind the wheel impaired, and we know that marijuana impairs judgment, reaction times and awareness."

Roadside Survey fact sheet

Crash Risk Study fact sheet