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The Confusing Science of Stoned Driving

With marijuana legalization taking root in two states and set to spread like a grass fire in the near future, the question of how to effectively determine impairment has never been more urgent—or controversial.

Let's start with a simple, unassailable premise:

Operating a 3,000-plus pound motor vehicle on public roads while dangerously impaired on any psychoactive substance—from opium to OxyContin—is not only illegal, it's immoral.

The trick, when it comes to marijuana and driving, involves determining what exactly “dangerously impaired” means. Ask a few average all-American, pot-smoking teenagers, and they'll likely say there's no such thing. A recent survey conducted by insurance giant Liberty Mutual, for example, found that among teens who admitted to driving after consuming cannabis, more than 70 percent self-reported no negative effects whatsoever on their competence behind the wheel, including 34 percent who believed, however dubiously, that getting blazed was actually performance enhancing.


Meanwhile, at least ten states mandate severe penalties for any trace of THC in a roadside drug test, even inactive metabolites that remain detectable up to a month after use. In February, the Arizona State Court of Appeals went so far as to reinstate the conviction (overturned by a lower court) of a man charged with a marijuana DUI despite a blood test that proved he wasn't under the influence. An Orwellian decision that confirmed Arizona's zero-tolerance “legislative ban extends to all substances, whether capable of causing impairment or not."

The Michigan Supreme Court, on the other hand, just last week ruled that registered pot patients do enjoy a limited exemption from zero-tolerance prosecutions. Although driving while impaired remains illegal in all circumstances, the court decided that the mere presence of THC in a patient's blood or urine should be viewed as a legal form of “internal possession.” Since state law allows patients to carry up to 2.5 ounces of cannabis, and currently offers no clear guidelines to determine impairment, the justices urged lawmakers to move quickly toward creating a universal standard similar to the one used for alcohol.

Which sounds imminently reasonable, until you start to look at the science of stoned driving, including a significant number of peer-reviewed studies with results that range from confusing to contradictory. For example, according to a widely reported meta-analysis published in the British Medical Journal, marijuana use within one hour of driving doubles the risk of a serious automobile accident. But a later review in the journal Accident Analysis and Prevention claimed those findings were overblown, likening increased risk to the use of antihistamines or penicillin. While a 2009 report from the US National Library of Medicine concluded that though “cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect.”

And if all that isn't confounding enough, wrap your mind around one last study that shows widespread use of medical marijuana actually produces a major improvement in public safety. Mostly because legal access to cannabis leads a significant number of drivers to smoke buds instead of drink Bud, a relatively safer choice that ends up saving a lot of lives.

Read the rest over at Motherboard.