A marijuana cultivation facility in Las Vegas, Nevada. Photo: Getty

4/20 is pretty much the favorite number of the internet (and of Gizmodo’s Slack chat), thanks in part to the growing societal acceptance of weed. Nearly two-thirds of Americans, and maybe even a majority of Republicans, now support its legalization. But a new study published today in JAMA Internal Medicine suggests that America’s boisterous celebration of April 20 these past two and a half decades may have had a relatively small but tragic consequence: more fatal car crashes.

Researchers studied data on fatal car crashes in the U.S., taken from the National Highway Traffic Safety Administration. They went as far back as 1992, two years after the holiday of 4/20 was unofficially christened by Grateful Dead fans, and a year after the magazine High Times popularized it to potheads everywhere. They looked specifically at crashes that occurred after 4:20 pm on April 20, then compared them to crashes during the same time period a week before and a week after the date.

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From 1992 to 2017, there were nearly 900,000 fatal crashes in the U.S. that involved 1.3 million drivers and led to nearly a million deaths, the researchers found. And on April 20 each of those years, the likelihood of a fatal crash was about 12 percent higher following 4:20 pm than it was during the other days they looked at, with a total of 1,359 drivers involved in car crashes those afternoons since 1992. Ultimately, the researchers estimated the holiday might have contributed to 142 more deaths in the past 25 years.

“Although the vast majority of Americans do not celebrate 4/20, the observed association was comparable in magnitude to the increase in traffic risks observed on Super Bowl Sunday,” the researchers wrote.

As might have been expected, crashes after 4:20 p.m. on April 20 were more likely among younger drivers, a group already more prone to car accidents, while any additional risk practically vanished for middle-aged drivers. And the biggest net increase of crashes was seen in New York, Texas, and Georgia; Minnesota, meanwhile, was the only state to have a lower relative risk during that time. Interestingly enough, though, there was no change in the increased risk seen during the more recent period of 2004 to 2016, lead author John Staples, a clinical assistant professor of medicine at the University of British Columbia in Canada, told me via email.

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For context’s sake, it’s worth noting that almost a third of fatal crashes involve legally drunk drivers, while half involve drivers who have drank any alcohol. And it’s estimated that around 10,000 crashes alone are caused by drunk drivers annually. Pot certainly causes fewer traffic deaths than alcohol, even if its true toll is still debated over. But researchers like Staples are also worried that these entirely preventable deaths will become more common the more pot is legalized and widely used.

“Although there is broad consensus among researchers that drug impairment increases crash risk, a surprising number of people drive shortly after using marijuana,” said Staples, citing research showing that 40 percent of regular pot users have driven less than two hours after a toke session, while 15 percent have mixed in alcohol to boot. That same study found only half of pot users think it’s dangerous to drive stoned. “A first step is to get the message out that no-one should drive while high,” he said.

At the same time, Staples doesn’t want to drum up any sort of reefer madness.

“While drugged driving is a problem that’s getting lots of attention, we still need to work on addressing more common problems like drunk driving and distracted driving,” he said. “It will be helpful for all drivers to remember to buckle up, put the phone away, don’t speed, stay sober, and don’t drive high.”

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[JAMA Internal Medicine]