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No One Knows How Stoned Is Too Stoned to Drive

A new study confirms what a lot of people already know: Treating THC like alcohol when it comes to DUIs has no basis in science.

Image by Lia Kantrowitz

Let's get something out of the way first: It's bad to drive while you're stoned. You probably already know this, being a young-ish person in a world with an ample supply of both cars and weed. The paranoia, the tendency to focus too much or not enough, the slight inaudible sort of buzzing that settles around you—all of that can add up to produce a horrible accident. Common sense and studies both tell us driving while high is stupid.


Common sense and other studies also tell us, however, that stoned drivers are much, much less dangerous than drunk drivers. And there's the rub: As weed use becomes decriminalized and legalized across the country, what sort of laws should we put in place to prevent marijuana-related DUIs? Several states, including Washington, Colorado, and Ohio, have adopted regulations that mirror the ones in place for booze, meaning if you have a certain amount of THC in your bloodstream you cannot legally operate a vehicle. Unfortunately, those laws are basically nonsense, according to a new survey released by the AAA Foundation for Traffic Safety.

The authors of the report examined records of thousands of drivers who had tested positive for THC after being pulled over. Most had alcohol in their systems along with THC (a big no-no for drivers), but among those who were solely on weed, the levels of coherence varied widely. Some who came in under the legal limit were obviously unfit to be behind the wheel, while others who were stoned out of their gourds on paper appeared to be unaffected.

The fact that weed affects different people in vastly different ways isn't surprising to anyone over 15. Some smokers are all like, "Hey let's go play basketball and talk to my parents," while others are more, "Actually hold up I want to sit very quietly over here on this bench for several hours." The disregard by lawmakers for this universal pot truth is frustrating, but not terribly surprising.


A local news channel in Seattle even dramatized this phenomenon pretty effectively a couple years ago in the wake of the 2012 legalization of recreational pot. It conducted an experiment in which three people—one who only toked occasionally, one who was a weekend pothead, and one "heavy daily user"—sucked down a third of a gram each and got behind the wheel of a car to take a supervised driving test, then smoked some more and repeated the process. Two of them flunked after nine-tenths of a gram, as you'd expect (one of them nearly hit the cameraman), but the heavy user, a 27-year-old girl named "Addy," who was three times above the legal limit for THC when she showed up to smoke some weed on TV, was "excited about being high and behind the wheel" and actually wasn't a bad driver. It wasn't until she had smoked 1.4 grams that she started having problems, backing up into a cone. (Addy became a minor internet celebrity as the "delightful driving test stoner.")

It does make sense for politicians in states set to legalize weed to worry about an uptick in stoned drivers—the AAA Foundation also found that more drivers involved in fatal crashes in Washington had THC in their blood after the state made recreational pot legal. But figuring out how high is too high to get behind the wheel is hard, not least because regular field sobriety tests don't really work on stoners—"A 21-year-old on his first bender and a hardened alcoholic will both wobble on one foot. But the same is not necessarily true of a driver who just smoked his first joint and the stoner who is high five days a week," is how a 2014 New York Times story put it.

The AAA Foundation believes that officers should use specialized tests to determine if drivers are impaired, not simply rely on how much THC is in someone's blood (traces of marijuana can linger in the bloodstream for days or weeks after smoking, making some states' outright bans on any THC in drivers' systems obviously absurd). The foundation also recommended that the existing state laws concerning THC concentrations be scrapped as they don't rely on science—but other people have been beating that drum for a while, with little effect. Meanwhile, a bill that would set THC blood level limits for drivers is advancing in Maine, and Michigan's legislature is also looking into the issue. It seems unlikely that another study telling us what we already know is going to stop lawmakers from making these choices—more likely, this is going to be a fight that pot advocates will have to wage after they finally achieve countrywide legalization.

Follow Harry Cheadle on Twitter.