The overall increase amounted to 142 additional deaths over the 25-year study period. Impairment with alcohol and drugs increases the risk of crash.
The researchers analyzed USA government data on fatal traffic accidents from 1992 - shortly after 4/20 was popularized as a pot holiday in High Times magazine - through 2016.
The research does not consider whether cannabis played a direct role in the April 20 fatal crashes, Staples said, because a police drug report - which combines lab tests and the impression of the officer - was included in the Fatality Analysis Reporting System for only about a third of the 1,369 drivers involved in a deadly April 20 accident over that time period.
"My message to the public is: Don't drive high", Staples told VICE.
However, marijuana legalization advocates point out that this study doesn't pin the blame directly on marijuana use.
"The data in this paper does not identify whether any of the drivers involved in motor vehicle accidents on the day in question were either under the influence of marijuana or responsible for the accident", Paul Armentano, the deputy director of NORML, the nation's largest cannabis law reform group, told HealthDay.
Scientists have found that there are more deadly road collisions than normal on April 20, the unofficial celebration for marijuana consumers in the United States known as 4/20.
Legislators and voters should keep this in mind as they consider new laws that would make marijuana more freely available 365 days a year, Staples and Redelmeier warned.
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Marijuana is legal for medical use in most US states and in Canada; recreational use has been legalized in nine states plus Washington, D.C. and is expected to become legal in Canada in July. The program runs through April this year.
Results: The risk of a fatal traffic crash was higher after 4:20 p.m. on April 20 compared with the identical time on other days used for comparison. The study, authored by doctors from the University of British Columbia and the University of Toronto, hypothesizes that fatal auto accidents are more likely following 4/20 celebrations in the United States.
It's possible people at the big 4/20 events also are drinking or taking other drugs, and that's what increases fatal crash risk, Staples added. Redelmeier, a professor of medicine at the University of Toronto, previously studied a spike in traffic fatalities on Super Bowl Sunday.
The study, published Monday in the medical journal JAMA Internal Medicine, analyzed data from the U.S. National Highway Traffic Safety Administration beginning in 1992 and found a 12-per-cent increase in the risk of a deadly accident after 4:20 p.m. on April 20 - an effect the two authors likened to the proven drop in road safety on Superbowl Sunday. "Unfortunately, we don't have a good way of knowing for sure which of those are contributing".
Their advice speaks to how easily Americans accept alcohol as a part of life and leisure, while marijuana is still treated with suspicion - even though the Super Bowl data are far worse than those outlined in the 4/20 study. Cannabis consumers will continue to use marijuana regardless of their state's legalization status.
"Nor are they jurisdictions known for having high-profile, well-attended marijuana-related events on April 20", Armentano said. "Just like alcohol and other drugs, cannabis use increases the risk for vehicle crashes".
Each year on April 20th at 4:20 p.m., thousands of cannabis aficionados come together to light up and celebrate what has become the "high holiday" for pot smokers.
"That might be an indication of where we should focus our efforts", he said of younger people driving high. "And it's definitely worth looking into the 4/20 phenomenon a little more deeply to figure out what it means in terms of traffic deaths".