Driving while intoxicated is unsafe, irresponsible, and uncouth. It’s also pervasive, which is why there are laws prohibiting this behavior.
These laws work: fatal crash rates today are less than half their early-1970s peaks, a trend reversal that suggests punitive drunk-driving laws are one of the most successful public-health interventions of the latter half of the 20th century.
But what about weed?
In the marijuana legalization era, public-safety officials and lawmakers have struggled with the question. Despite clear evidence cannabis and alcohol are very different substances—metabolized differently, affecting the body differently—there’s been a clear impetus to treat stoned driving like drunk driving.
However, so far, “cannabis breathalyzers” as well as “marijuana DUI” limits have failed scientific and legal tests. You can’t judge how stoned someone is by measuring their breath, saliva, or their blood; levels of THC metabolites in the body just don’t correspond with intoxication the way blood-alcohol does.
So what can you do? You could wait about three and a half to four-and-half hours after smoking marijuana. After that, you’re likely to drive just as well you would have before consumption, according to recent original research conducted by the University of California, San Diego and published earlier this year in the Journal of the American Medical Association.
In a two-year-long study, researchers observed the driving abilities of “191 regular cannabis users,” most of whom smoked at least every other day. Study participants were given joints filled with cannabis with 5.9 percent or 13.4 percent THC, or bunk (placebo) weed with no THC at all. They were told to smoke as much as they wanted to reach a “normal” level of intoxication before hopping into a driving simulator.
Researchers made several key findings, according to Thomas Marcotte, a professor of psychiatry at UCSD and the study’s lead author. One confirmed what other studies already suggested: that blood THC concentration was a very poor indicator of driving performance.
“We know from previous research there’s not a good correlation,” Marcotte said in a recent interview. “But it was sort of compelling how much of a lack of a relationship was there.”
Another was that while study participants agreed that they were too high to drive after immediately smoking, at about the 90-minute mark, people started to feel like they were fine when they weren’t. They drove just about as poorly as they did after smoking.
“At 90 minutes, people started feeling better,” Marcotte said. “They were more willing to go on the road but their performance did not change.”
But at about the three-and-a-half hour mark, their driving performance showed only a “borderline difference” between performance while sober. An hour later—four-and-a-half hours after consumption—there was “no differences” between post- and pre-smoking performances, according to the study results.
This doesn’t necessarily mean that cannabis users are mostly fine or good to go after four and a half hours, Marcotte cautioned.
But as far as rough rules of thumb go, it’s something to go by. Obviously, if you feel too high to drive, you probably are. But be aware of that 90-minute false sense of security—and beware.
“It’s a little challenging for the [cannabis] consumer to know when it’s okay to drive,” Marcotte said.
“I wish we had solid advice,” he added. “It’s not very satisfying, but that’s where we’re at right now.”
With these results in mind, instead of marijuana breathalyzers, what might work better to maintain road safety in the marijuana legalization era are devices that accurately measure how recently someone had used cannabis, Marcotte suggested. Whether that standard would make effective legislation or hold up in the courts are separate questions.
Until then, cannabis users could follow an even simpler diktat: if you feel you’re too high to drive, you probably are. And when you think you’re stating to feel fine, wait a little bit longer.