One subject on the minds of the public and policy makers as marijuana becomes re-legalized is the topic of so-called “stoned driving.” Fears of legal marijuana opening the floodgates to a rash of stoned mayhem on the freeways led to Washington’s legalizers including a per se limit (you’re guilty of DUID if over 5ng of THC per milliliter of blood) in their initiative. While Colorado’s legalizers didn’t add a stoned driving limit in their amendment, political pressure culminated in adoption of a permissible inference limit (you’re probably guilty of DUID if over 5ng of THC, but you can argue your innocence) by statute.

These stoned driving limits are a solution in search of a problem, as roadway accident and fatality stats have never been lower. Besides, they are unscientific and unjust, as “driving impairment cannot be predicted by prevailing [blood] plasma concentrations of THC or THC-COOH [an inactive marijuana metabolite].” That’s the conclusion of a study conducted by the US Government’s own National Highway Traffic Safety Administration … 21 years ago!

The November 1993 report, “Marijuana and Actual Driving Performance,” was prepared for the US Department of Transportation and is available online here. The authors, Hindrik Hobbe and James O’Hanlon, working at the University of Limburg in Maastricht, The Netherlands, performed one pilot and three actual driving studies, in highway and dense urban traffic conditions, and found that “THC’s adverse effects on driving performance appear relatively small” and “drivers under the influence of marijuana retain insight in their performance and will compensate where they can, for example, by slowing down or increasing effort.”

It’s not that marijuana isn’t impairing behind the wheel -- nobody’s suggesting you rip a few bong loads before heading out on the road. “The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road,” the authors wrote, “but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol.” Basically, driving high means it is harder for you to maintain lane position.

However, the authors are clear about the much more significant driving danger posed by alcohol, noting that “THC… has relatively minor effects on driving performance, certainly less than blood alcohol concentrations (BACs) in the range of 0.08-0.10.”  The authors found that marijuana’s effects on that steady lateral position measurement “were equivalent to those associated with BACs in the range of 0.03-0.07,” or, in other words, a drinker who’s not automatically presumed to be too impaired to drive.

“THC is not a profoundly impairing drug,” Hobbe and O’Hanlon wrote. “It does affect automatic information processing, even after low doses, but not to any great extent after high doses. It apparently affects controlled information processing in a variety of laboratory tests, but not to the extent which is beyond the individual's ability to control when he is motivated and permitted to do so in real driving.”  The relevance of laboratory driving simulations was also doubted, with the authors noting “It appears that performance is more affected by THC in laboratory than actual driving tests.”

They further highlight the difference between the self-awareness of drunk vs. stoned drivers, noting, “Alcohol impaired driving performance according to the driving instructor but subjects did not perceive it; marijuana did not impair driving performance but the subjects themselves perceived their driving performance as such.” This is that well-known tendency of marijuana-using drivers to slow down, leave more distance between cars, and brake earlier.

While anti-legalization crusaders will tout studies saying marijuana doubles your risk of car crash, they always leave out the fact those same studies show about the same doubling of risk for drivers with antihistamines or penicillin in their systems. Hobbe and O’Hanlon knew that in 1993, writing, “THC's effects after doses up to 300 μg/kg never exceed alcohol's at BACs of 0.08; and, were in no way unusual compared to many medicinal drugs.”

We’ve painted ourselves into a bit of a corner by insisting we “regulate marijuana like alcohol,” because now the public wants to know where the equivalent of a breathalyzer is for marijuana. By pointing out that our own government for 21 years has accepted that “it appears not possible to conclude anything about a driver's impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample,” maybe we can extinguish some of these stoned driving myths.

 

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