New study shows no link between marijuana use and car accidents

Calley Hair | Washington State University

The effect of marijuana use on drivers just became a little less hazy – and not in the way legislators or medical experts might expect.

A new study from the National Highway Traffic Safety Administration found that consuming marijuana does not elevate the crash risk of the driver, a result that’s leaving road safety decision-makers wanting more information.

“There’s a pretty clear accelerating trend of marijuana use among drivers,” says Gordon Trowbridge, communications director at the NHTSA. “There’s more and more of this on the highway and it’s something we know relatively little about.”

The study looked at more than 3,000 drivers involved in crashes over a 20-month period in Virginia Beach, Va., measuring which substances -- if any -- were in their systems at the time of the accident. The study also included 6,000 control drivers in the same area over the same time period who were not involved in any accidents.

“It was the most closely controlled study of its kind that has ever been conducted,” Trowbridge says.

Unsurprisingly, the results showed that drivers with alcohol in their systems were far more likely to crash. Adjusted for age and gender, the study found a driver with a blood alcohol content of .08, the legal limit, is four times as likely to crash as a driver with no alcohol in his or her body. When BAC hits .15, drivers are 12 times as likely to get into an accident.



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What’s more unexpected, the odds -- adjusted for age and gender -- of a driver with THC in his or her system crashing are only 5% higher than those of a sober driver.

“Marijuana is complicated. This study is a very important part of beginning to learn the factors we need to pay attention to,” Trowbrigde says. “We know far less about marijuana at this point than we do about alcohol.”

While the study indicates a difference between how THC and alcohol actually intoxicates drivers, they are treated the same way by police officers on the road.

“If we suspect that they are intoxicated (with marijuana) and driving, then we handle it like we would alcohol where we put you through field sobriety tests,” says Commander Chris Tennant of the police department in Pullman, Wash., where marijuana use is legal for residents 21 years of age and older.

Tennant says that Pullman police tend to find drivers under the influence of marijuana only after they’ve been pulled over for a different traffic violation.

“It’s only after that that we detect there’s any intoxication problem,” Tennant says, adding that there’s no particular pattern to the road mistakes these drivers make.

There is, however, a specific pattern to the drivers caught with marijuana in their systems: most of them are under the influence of other drugs, usually alcohol, Tennant says.

“We’re a small community. Obviously when it comes to a DUI, we’re a target community because a very large percentage of our population is college-age,” Tennant says. “Our experience with marijuana here in Pullman is that it’s pretty rare that we run into someone just using marijuana.”

While it’s tempting to deduce that marijuana is harmless to drivers, users should think twice before lighting up and hopping behind the wheel. Trowbridge emphasized a very important distinction highlighted by the study -- just because the drug is negligible to accident rates does not mean it is negligible to driving safety.



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“We know from previous simulator studies that marijuana does have an effect that degrades the ability of drivers,” Trowbridge says. “It is good to be cautious about drawing sweeping conclusions.”

Looking forward, it’s far too soon to start guessing how the study might influence action on the legislative floor, Trowbridge says.

The research is far from complete for the NHTSA, which is currently working on a study alongside the state of Washington to understand the effect of legalization on traffic safety. Trowbridge says the NHTSA also partnered with the National Institute on Drug Abuse to better understand the effect of marijuana on individual driver performance.

“We’re just in the early stages of this process,” Trowbridge says.

The NHTSA’s ultimate goal is to provide lawmakers with the best possible information to make laws about traffic safety, information that has previously been lacking with regards to marijuana and the risk it poses to drivers.

“The alcohol equation is somewhat simpler than the drug equation,” Trowbridge says. “We think it’s important for those policymakers to have much more data in hand to make those decisions.”



Calley Hair is a student at Washington State University and a spring 2015 USA TODAY Collegiate Correspondent.

This story originally appeared on the USA TODAY College blog, a news source produced for college students by student journalists. The blog closed in September of 2017.