Q: I know all about the dangers of drunk driving. But is driving while high on marijuana so dangerous?
Drinking of any kind is of course a bad idea, but smoking a joint or sipping an edible before getting behind the wheel can carry distinctive risks. That, experts say, is because of the specific ways marijuana affects the brain, and the fact that there is no standard dose for a federally criminalized drug.
With alcohol, there are universally accepted amounts for what makes up a single drink: 12 ounces of regular beer, five ounces of wine, and 1.5 ounces of spirits. There is no agreed-upon equivalent for cannabis, said Marlene Lira, a senior research manager with the Clinical Addiction Research and Education Unit at Boston Medical Center. This makes it difficult for consumers to measure how much they have actually consumed and assess how much cannabis is too much before hitting the road.
Over the past decade, there have been an increasing number of car accidents involving cannabis and recreational use of the drug continues to rise. A recent analysis of U.S. public safety data showed that from 2000 to 2018, the percentage of cannabis-involved motor vehicle fatalities more than doubled from nine percent to about 22 percent. In contrast, the percentage of fatalities under the influence of alcohol remained about the same during this period.
While there’s a lack of data comparing the dangers of drink-driving and drink-driving, the research does suggest that marijuana may be less likely to lead to fatal car accidents than alcohol. In a 2017 study of more than 4,000 drivers from a police database in France, researchers found that drivers under the influence of alcohol were about 17.8 times more likely to be responsible for fatal car accidents than drivers who were sober, while drivers under the influence of alcohol were about 17.8 times more likely to be responsible for fatal car accidents. Influence of alcohol marijuana was 1.65 times more likely to cause fatalities. A 2007 study of US road safety reports estimated that drivers who used marijuana were 1.83 times more likely than sober drivers to be involved in a fatal accident.
Studies like this one are often based on drivers being drug tested after car accidents. But because cannabis can linger in adipose tissue for up to 28 days, a positive drug test doesn’t necessarily mean a driver was high at the time of driving. This can lead to inaccurate measurements of how cannabis actually affects a person’s driving ability, said Dr. Godfrey Pearlson, a neuroscience professor at Yale University who has researched marijuana and car safety.
It is also difficult for marijuana users to predict exactly how they will be affected and for how long. If you smoke a joint or take a bong, you’ll feel high within minutes and return to a basic level after three or four hours, said Dr. Pearlson. If you opt for a pot of brownie or a gummy, the high will take longer to kick in as the marijuana in edibles needs to be absorbed in your gut and metabolized by your liver. And it’s hard to predict how long that high will last, said Dr. Pearlson.
“If someone takes an edible, the classic scenario is they say, ‘Hey, I’m not feeling this,’ and then 40 minutes go by, and they get in a car and start driving,” said Dr. Collin Reiff, a psychiatrist at NYU Langone Health, in New York City.
THC, the intoxicating compound in cannabis, hinders a number of behaviors associated with driving, including the ability to swing between lanes. It also slows reaction times in general, Ms Lira said.
The psychoactive effects of cannabis can also make driving dangerous. The paranoia some people experience while high can trigger a panic attack while on the go, said Dr. Reiff. Weed can also create an altered sense of time, the untidy feeling that everything around you is moving slower than it actually is. “There’s the joke: why were you arrested?” he said. “If you drive too fast, it’s because of alcohol. If you go too slow, it’s because of cannabis.”
Cannabis also clouds the perception of what is safe and what is not. In a randomized study of 191 regular cannabis users (people who used the drug an average of 16 times a month), researchers instructed participants to smoke THC cigarettes or placebos until they got as high as they wanted over a 10-minute period of time. at least four puffs. Thirty minutes later, and during the day, they were led to a series of driving simulations. Before each simulation, researchers asked study participants to rate how tall they were and how badly they had to ride. After that initial 30-minute interval between smoking and driving, just under half of the study participants who smoked THC said they felt safe to drive in their current state. But after 90 minutes, nearly 70 percent thought they were okay to drive, even though the THC group performed just as poorly (and worse than the placebo group) in the simulations.
“There’s a disconnect — after a while they feel it wears off,” said Thomas Marcotte, a professor of psychiatry at the University of California, San Diego, who led the study. “But actually you are not necessarily a good judge at that moment. You still don’t have to hit the road.”