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Driving After Taking Any Form of Marijuana Is Unsafe Skip to content

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Driving While High on Pot Is Not Safe

What’s legal on the roads varies by state

A man's hand reaching for car keys on a blue counter next to a marijuana vape pen

PHOTO ILLO: YASU + JUNKO, Getty Images

En español | Get paid to drive high? When news of a University of California at San Diego cannabis-and-cars study got over 750,000 views on Twitter in July 2018, lead researcher Thomas Marcotte suddenly had plenty of volunteers. “We had a waiting list of more than 2,000 people,” he says. “We're grateful for the interest. Driving under the influence of marijuana is a complex subject. We don't even know if the standard recommendation to wait three or four hours is accurate.”

Not everyone is waiting for answers. The AAA Foundation for Traffic Safety estimates that nearly 15 million Americans hit the road under the influence of weed in the 30 days surveyed. In a 2019 study of 790 medical marijuana users released earlier this year, 1 in 5 admitted to having driven “very high” and 56 percent said they had gotten behind the wheel within two hours of a dose.

Did it affect their driving? “It's clear that cannabis can impair driving. Part of why it's complicated: It depends on method of ingestion, dose and experience, etc.,” says Marcotte, codirector of the U.C. San Diego's Center for Medicinal Cannabis Research. In fact, research shows that marijuana interferes with vital vehicle-handling skills, including reaction time, mental focus, the ability to stay in your lane, even your sense of time and distance. Cannabis use boosts accident risk two- to 14-fold. It also doubles the odds of a fatal collision.

The dangers of driving while high are generally accepted, but whether a driver is “high” is still a matter of debate. Marcotte explains that driving high isn't like driving drunk; there's no easy roadside too-high-to-drive test like a Breathalyzer. There's no consensus on how much cannabis it takes (or how potent it has to be) to make you a driving risk, either. The amount of THC in the blood, breath or saliva, he says, does not closely match with impairment. It also depends on people's experience with cannabis and whether they've developed a tolerance to the effects.

“THC isn't like alcohol,” Marcotte says. “Blood levels of THC don't correspond well with how impaired a driver is. Unlike alcohol, which stays in your bloodstream for hours and dissipates in a linear manner, THC is fat friendly. If you smoke or vape, THC levels spike in your blood, then drop significantly within 30 to 60 minutes as the THC is absorbed into fat in your body — including in your brain, which has a high fat content. So THC levels look low as the euphoria and brain effects of cannabis persist. Adding to the difficulty, low levels of THC can linger in the blood for hours, days or weeks in frequent users but may not affect driving."

It is even harder, Marcotte notes, to “relate THC blood levels and impairment in edibles, where the brain effects take longer to kick in and last much longer than smoking or vaping.”

The laws

All states have driving-under-the-influence-of-drugs laws, but they don't match up. In 12 zero-tolerance states (including Arizona, Georgia, Iowa and Wisconsin), virtually any amount of THC in the body is illegal. A handful of others (including Illinois, Montana, Nevada, Ohio and Washington) set blood THC limits that generally range from 2 to 5 ng/ml. Colorado has a similar rule but requires extra proof of impairment. In the rest, marijuana-impaired driving is simply considered unlawful, but it is extremely hard to measure and prove. That may soon change, though. Marcotte and his research team recently completed a study, authorized by the California state legislature, in which 180 volunteers ages 21 to 55 inhaled high-THC, low-THC or zero-THC cannabis smoke, then drove in a simulator and took a battery of blood, breath and saliva tests over six hours. Their memory, ability to estimate time, and body sway were also checked with a tablet computer, which could lead to new roadside tests for stoned driving. “Field sobriety tests for alcohol don't work so well with marijuana. People don't stagger and lose balance the same way,” Marcotte points out.

Best practice

More studies are on the way that will make it easier to determine how soon it is safe to drive after ingesting cannabis. In the meantime, it's safest to avoid mixing marijuana with driving.

More on Medical Marijuana

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