U.S. traffic fatalities rise dramatically on the day pot smokers celebrate as “Weed Day.” In the quarter-century since High Times magazine proclaimed April 20 a time to light up and smoke marijuana, traffic fatalities have spiked 12 percent on that date, compared to one week before or after, a new study shows.

“This was such a great natural experiment to examine the risk of cannabis intoxication,” said lead author Dr. John Staples, an internist and researcher at the University of British Columbia in Vancouver.

Though the study could not assess whether marijuana-intoxicated drivers caused the surge in vehicle deaths on the counter-cultural “High Holiday” dubbed “4/20,” they appear to be the most likely culprits, Staples said in a phone interview.

Since High Times popularized the date in a story the magazine published in 1991, thousands of Americans have been celebrating the intoxicating properties of cannabis on April 20, the authors write in JAMA Internal Medicine.

Five San Rafael High School students claim to have coined the term “4/20” after regularly meeting at 4:20 p.m. in 1971 to search for a patch of pot plants in a nearby forest.

The increased risk of fatal traffic crashes on April 20 was comparable in magnitude to the increased traffic risks observed on Super Bowl Sunday, the authors write. Redelmeier, a professor of medicine at the University of Toronto, previously studied a spike in traffic fatalities on Super Bowl Sunday.

All U.S. states prohibit driving impaired by marijuana, said Jennifer Whitehill, a professor of health promotion and policy at the University of Massachusetts Amherst, who was not involved with the study. Organizers of 4/20 festivals should promote safe-driving measures, she said by email.

Nevertheless, Forman, who was not involved with the study, credited the report with creating “the opportunity to take a deeper dive into the research on marijuana and road safety to truly understand whether there is a link between marijuana use and crash risk, and the extent of that risk.”

Previous research has shown that opioid-overdose deaths and hospitalizations drop in states that legalized marijuana, she said by email.

“For policymakers implementing marijuana legalization, it is critical to invest in research that will help us understand the impact the policy change may have on traffic safety,” she wrote.

Whitehill also urged parents to discuss the dangers of impaired driving from all psychoactive substances, including marijuana. “As marijuana becomes legal in more states,” she said, “this type of conversation will become more necessary and, hopefully, more common as well.”