In the wake of relaxed marijuana laws across the country – including legalization of recreational use in Colorado – states are scrambling to figure out how the law should treat impaired drivers. But initial research on how marijuana affects driving has produced mixed results, further muddying the waters for road safety advocates.

“We see the legalization of marijuana in Colorado and Washington as a wake-up call for all of us in highway safety,” said Jonathan Adkins, executive director of the Governors Highway Safety Association, according to a recent report in the Christian Science Monitor.

“We don’t know enough about the scope of marijuana-impaired driving to call it a big or small problem. But anytime a driver has their ability impaired, it is a problem,” Adkins said.

Conflicting Conclusions in Academic Studies

Highlighting this knowledge gap are two recent studies that assessed similar data on marijuana-impaired drivers, but reached opposite conclusions.

Researchers at both Columbia University and the Pacific Institute for Research and Evaluation looked at drugged driver data collected by the National Highway Traffic Safety Administration (NHTSA) in its 2007 National Roadside Survey of Alcohol and Drug Use and compared it to data collected by states that drug test drivers involved in fatal crashes.

The Columbia researchers found that marijuana alone increased a driver’s fatal crash risk by 80 percent, while the Pacific Institute researchers concluded that drivers on pot alone were somewhat less likely than drivers with no drugs in their system to have been involved in a crash.

Dissimilar results such as these make the link between marijuana and crash risk “highly inconclusive,” according to Dr. Mehmet Sofuoglu of Yale University Medical School.

Sofuoglu testified at the trial of a New York teen who smoked marijuana before driving his Subaru over 100 mph and crashing, killing four friends. He blamed the accident on the tendency of male teenagers to take risks, not on what the prosecutor called “speed and weed.”

Marijuana has been shown to slow decision making and decrease peripheral vision, but Adkins told USA Today that stoned drivers typically drive more slowly and less aggressively. The biggest concern, he says, is “if you mix it with alcohol, it has a stimulative effect. It makes reckless drivers. It makes drivers take a lot of chances.”

The Columbia researchers found that a driver under the influence of alcohol and marijuana is 24 times more likely to die in a crash than a sober driver, while an alcohol-impaired driver is 13 times more likely to be in a fatal crash than a sober driver.

Arbitrary Limit?

Part of the problem for policy makers, however, is defining what is meant by marijuana “impairment.” THC can be detected in the blood up to seven days after use, making it hard to tell who is high on the drug and who simply has the presence of the drug in his or her system.

Colorado has set the threshold for driving under the influence of marijuana at 5 nanograms per milliliter of blood, but critics have said thresholds set by Colorado and other states are arbitrary.

“We know almost nothing about how [marijuana] affects driver performance at different concentrations in people’s bodies,” said Jake Nelson, director of traffic safety, advocacy and research for the American Automobile Association (AAA). “Does it matter if you consume a lot or a little? How quickly does the substance dissipate or break down in the body? We don’t know anything that we would like to know before we start to pass law.”

Driving while impaired by marijuana is illegal in Colorado, and drivers who exceed the intoxication threshold could be held liable for an automobile accident.

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