Everyone knows that it’s dangerous to drive drunk, but the public discourse around driving stoned is a bit fuzzier.
As cannabis transitions to mainstream acceptance in the U.S., the spotlight has often been on the benefits of legalization, and potential health upsides. What isn’t discussed as much is what happens when people smoke up and get behind the wheel—with catastrophic consequences.
The National Transportation Safety Board (NTSB) issued a safety alert about the dangers of driving while stoned last month, after it concluded an investigation that found cannabis played a role in a 2022 car crash that killed six Oklahoma high school students.
“We found that there was widespread misconception about the impairing effects of cannabis,” NTSB Office of Highway Safety director Robert Molloy tells Fortune. “We recognized that teenagers are especially vulnerable to the risks associated with marijuana-impaired driving due to their limited driving experience… I think we also do a poor job of talking about the effects of marijuana on driving.”
Several studies show that it’s unsafe to drive while high, revealing that cannabis significantly affects driver reaction time, decision making, and coordination. And experts say that while messaging around drunk driving has been robust, the infrastructure and education around stoned driving still has a long way to go.
What we know about driving while stoned
The most comprehensive study ever done on driving while high leaves little room for doubt: Marijuana consumption makes people worse drivers.
For a 2021 paper published in the Frontiers in Psychiatry journal, researchers came up with a representative dose of cannabis, then asked participants to complete a series of complex simulated driving tests: driving straight through unpredictable wind gusts, following at a safe distance behind another car that slows down and speeds up irregularly, and passing a car stopped on a highway while avoiding oncoming traffic in the other lane.
“We saw all sorts of different errors,” lead researcher Godfrey Pearlson, Yale University professor of psychiatry and neuroscience, and author of The Science of Weed, tells Fortune. “But in general, people’s reaction time is a lot slower—and significantly slower. And when it comes to the more complex decisions, they come much closer to being involved in an accident.”
Other studies have found that marijuana can also impair judgment of distance as well as coordination, according to the National Highway Traffic Safety Administration (NHTSA), both of which are essential behind-the-wheel skills.
Impairment, Pearlson’s study points out, is at its peak 20 to 40 minutes after inhalation (for smoking and vaping), and can last for up to two and a half hours, which is why “cannabis consumers should wait a minimum of three to four hours before attempting to drive,” he says; others have found it’s prudent to wait more like five hours.
Why the persistent belief that driving on weed is safe?
Some of the reluctance to talk about the dangers of driving while stoned may come from the backlash against how the U.S. has traditionally talked about weed in general, according to Pearlson.
“If you tell people for years and years, ‘If you take acid, you’ll stare at the sun and go blind,’ and that ‘cannabis will damage you genetically and cause you to be sterile,’ then people will stop believing it, because those things are not true,” Pearlson says. “So because of anti-drug propaganda being obviously false, people have tended to be skeptical about what they’re told about cannabis and believe that it’s therefore harmless.”
He adds that marijuana users also tend to be more aware than alcohol drinkers about how impaired they actually are, which can paradoxically make them think they’re capable of driving.
And Pearlson, who has led head-to-head comparisons of drunk driving versus stoned driving, says there is “some truth” to the idea that drunk driving is riskier. “Drunken driving is much more impairing than cannabis-impaired driving,” he says. One study conducted on data from France found that drunk drivers are 17 times more likely to be responsible for a fatal accident, compared to marijuana users who were 1.65 times more likely.
But, Pearlson stresses, the discrepancy between drunk and stoned driving doesn’t negate how dangerous it is to drive while high. “It’s just not true that cannabis-impaired driving is therefore harmless,” he says. “It’s just less impairing on typical doses.”
Keeping up with changing times
Cannabis is now legal in 24 states for recreational use, and in 38 states for medical use. But experts say that efforts to curtail driving under the influence of marijuana through education and enforcement have not kept up with the quickly shifting legalization landscape in the U.S.
A line from Pearlson’s 2021 study reads: “The authors believe that the legal cart is currently significantly ahead of the scientific horse.”
As it stands, says Pearlson, traffic stops that involve walking a straight line and reciting months of the year are geared more toward catching drunk drivers rather than high ones. “Those tests are great at picking up alcohol-intoxicated drivers, but completely inappropriate for cannabis-impaired drivers,” he says.
There is training available for police officers on how to detect high drivers specifically, including Advanced Roadside Impaired Impaired Driver Enforcement (ARIDE) and Drug Recognition Expert (DRE). But the NTSB knows it has a ways to go. In its 2022 study, the agency asked states to improve their drug testing capabilities by modifying laws on collection, screening and testing. There are currently tools that law enforcement can use to detect cannabis, including oral fluid roadside screening devices which test saliva, but those only test the presence of drugs, not impairment. And blood tests, which are often a second step for drivers suspected of driving while high, are ineffective in a different way.
Pearlson says the average time between pulling a driver over and having their blood drawn is 90 minutes. That’s not useful, he says, when the THC spike happens within the first five minutes of smoking.
“All they can detect is this low baseline level that may be because you smoked an hour ago or two weeks ago. No one can tell the difference,” says Pearlson.
While roadside testing has some kinks to work out, education around driving and cannabis should be a simpler path—and experts say it needs to catch up to where drunk driving has been for years. Molloy stresses education should be done both through warning labels on marijuana products and improved teen driver education about the risks of marijuana impairment.
Pearlson adds that public service announcements about “stoned driving” would help, but he stresses the need for more research—especially regarding increasingly popular cannabis concentrates and edibles, “both potent sources of THC and little-studied” in terms of driving impairment.
“Cannabis is much more complex than alcohol,” he says. But when it comes to driving, “no one treats it differently.”
