Drivers in the United States are more likely to be in a fatal traffic crash during the annual April 20 cannabis celebration, according to new research from the University of British Columbia.

"One-fifth of Americans now live in states that have legalized recreational cannabis, and legalization is set to occur for all Canadians in July 2018," said lead researcher Dr. John Staples, a clinical assistant professor of medicine and scientist at UBC's Centre for Health Evaluation and Outcome Sciences. "We hope that legalization doesn't lead to more people driving while high."

Along with University of Toronto professor Dr. Donald Redelmeier, Staples examined 25 years of National Highway Traffic Safety Administration data on all fatal traffic crashes in the United States. They compared the number of drivers involved in fatal crashes after 4:20 p.m. on April 20 with the number of drivers involved in fatal crashes during the same time intervals on control days one week earlier and one week later.

The investigators found that April 20 was associated with a 12 per cent increase in the risk of a fatal traffic crash. Among drivers younger than 21 years of age, the risk was 38 per cent higher than on control days. The overall increase amounted to 142 additional deaths over the 25-year study period.

Since the 4/20 holiday was first popularized in 1991, annual events in Denver, San Francisco, Vancouver, and other cities have grown to include tens of thousands of attendees. It isn't known how commonly drivers get behind the wheel while high on 4/20, but a 2011 study of U.S. college freshmen found 44 per cent of cannabis users drove soon after consuming marijuana in the month prior to the survey. Only half of cannabis users in the 2017 Canadian Cannabis Survey thought cannabis use affected driving.

"Assuming fewer than 10 per cent of Americans drive while high on April 20, our results suggest that drug use at 4/20 celebrations more than doubles the risk of a fatal crash," said Redelmeier.

Staples and Redelmeier hope that authorities will respond to these results by encouraging safer 4/20 travel options, including public transit, rideshares, taxis and designated drivers. The investigators also note that cannabis retailers and 4/20 event organizers have an opportunity to serve their customers and save lives by warning users not to drive while high.

As Canada and other places move toward legalization, Staples says it's also important to employ multiple strategies to reduce driving under the influence of drugs throughout the year.

"Driving is a potentially dangerous activity," Staples said. "Improving road safety requires both policymakers and drivers to make smart decisions. If you're going to get behind the wheel, buckle up, put the phone away, don't speed, stay sober and don't drive high."

The study was published today in JAMA Internal Medicine. The research was supported by Vancouver Coastal Health Research Institute, the Canadian Institutes for Health Research, and the Canada Research Chair in Medical Decision Science.