There is little evidence to suggest that fatal car accidents increase on April 20 (4/20), an annual holiday that celebrates marijuana, according to new research forthcoming in Injury Prevention.

As states across the country move to legalize medical and recreational marijuana, questions about the impacts of these policy changes – including on driving and rates of traffic accidents – are cropping up, said Sam Harper, associate professor of epidemiology at McGill University and one of the study authors.

“There’s a lot of dynamic action happening in the cannabis policy arena,” Harper said.

Prior research has indicated an increase in traffic fatalities on April 20. A research letter published in JAMA Internal Medicine in April 2018 looked at the relationship between April 20 and fatal traffic crashes in the U.S. Researchers looked at data for 25 consecutive years — from 1992 to 2016 – and compared the number of fatal crashes that happened on April 20 after 4:20 pm to the number of crashes that happened in the same time frame on control days — the same day of the week — one week before and one week after April 20.

They found “a 12% increase in the relative risk of a fatal traffic crash after 4:20 pm on April 20 compared with identical time intervals on control days.”

The research letter spurred Harper’s interest. He wanted to build on the research by comparing the impact of 4/20 on fatal traffic accidents with other risky days.

“We were motivated to look at this in more detail and see what we could find if we did a little bit more testing,” he explained, adding that there is significant day-to-day variation in fatal traffic accidents.

So Harper and co-author Adam Palayew expanded the analysis. In addition to comparing fatal traffic accidents on April 20 with single control days before and after the holiday, they looked at data for control days two weeks before and after. They also compared the number of fatal traffic accidents on April 20 to other known risky traffic days, such as the Fourth of July and Labor Day. Additionally, they analyzed the variation in fatal traffic accidents for every day of the year, “to put 4/20 in the context of other daily variations in traffic crashes,” Harper explained.

The key takeaway? “There is very little evidence that the [4/20] effect was observable over time,” Harper said.

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