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Reminder: We Still Don’t Know How Stoned Is Too Stoned to Drive

It’s not like measuring for booze, but experts still don’t know quite how to measure it.

Being stoned is not all like being drunk, but that hasn’t stopped law enforcement from trying to apply the same yardstick to testing for drugged impairment. An in-depth look at the state of the driving-while-high by the Marshall Project is a reminder that not only do the laws on stoned driving vary by state, but many of them still make no reliable sense.

“We take for granted that not being able to walk a straight line or stand on one leg means that you’re drunk, and that being drunk means it’s unacceptably dangerous to drive,” writes Beth Schwartzapfel. “But there is no clear scientific consensus when it comes to smoking pot and driving. And few of the tools police officers have long relied on to determine whether a driver is too drunk to drive, like a breathalyzer, exist for marijuana.”

So here’s the basic situation, according to Schwartzapfel’s research: The typical experience of being stoned is on par with a blood alcohol level of anywhere from .01 to .05, perfectly legal for driving in every state in the union. One researcher quoted said driving stoned at this level is much like the difference between driving during the day versus at night.

But even saying it’s “on par” with driving drunk is misleading: What makes this so tricky is that the courts have traditionally used how many drinks you’ve consumed, and their attendant BAC, to measure impairment — two things that tend to correlate. Drink more, drive shittier. That’s just not how pot works. “Scientists can’t say with confidence how much pot, in what concentration, used in what period of time, will reliably make someone ‘high,’” Schwartzapfel writes.

A recent study from American Automobile Association published at PBS came to the same conclusion: THC can’t be measured like alcohol. “As a result,” AAA said, “drivers who are unsafe may be going free while others may be wrongly convicted.” It’s not that smoking and driving is risk-free; it’s that other studies, like one from the National Highway Traffic Safety Administration in 2015, argue that there’s just no evidence that it increases risk as dramatically as driving drunk. PBS cites studies that found that crash risk for impairment goes like this: Being high doubles the risk of a crash, but talking on a hands-free cell phone, which is legal everywhere, quadruples it. Compare this with a 0.12 BAC level, which ups crash risk by 15 times.

There are other issues at play: THC in the bloodstream hits peaks and valleys based on how long it’s been since you toked, and if you smoke a lot of pot, you tend to have lingering amounts for days or weeks after the fact, even when you’re not high. And if you don’t smoke a lot of pot and then do, you’re going to be higher.

Some 18 states measure THC, though, and 13 of them would slap a driving-while-impaired charge on you for any amount in the bloodstream — zero tolerance. Five other states impose a “legal cutoff” of THC ranging between two and five nanograms per milliliter of blood. And the rest, 31 others, simply prohibit driving while high.

That typically involves a field sobriety test, but the Marshall Project’s prime example of how troubling this is involves driver Thomas Gerhardt, who, when pulled over in Massachusetts in 2013 by a state trooper, was able to count backward from 75 to 62 and reel off the alphabet from D to Q. He was not, however, able to walk nine steps and turn, nor to stand on one leg. He went to jail and was charged with a DWI, but his lawyer has recently argued that while a field sobriety test is great at finding drunks, it doesn’t prove that he was even high, much less too high to get behind the wheel.

What is to be done? Some researchers are looking at tracking software that might better determine a state of being way too stoned, or something breathalyzer-like, but that’s still about two years away, technologically speaking. At least until Gerhardt’s case—which may help point the way toward stoner clarity—is resolved this spring, it’s still dicey out there.