(Reuters Health) - 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.

“The simplest explanation is that some drivers are impaired by cannabis use, and these drivers are contributing to fatal crashes,” he said. “There should be very clear messaging to the public: don’t drive high.”

The impact of marijuana’s psychoactive effects on drivers is of particular concern given that six U.S. states now permit marijuana to be sold for recreational use to customers at least 21 years old.

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.

Staples and Dr. Donald A. Redelmeier analyzed U.S. National Highway Traffic Safety Administration fatality reports from 1992, the year after the High Times story, through 2016. Traffic fatalities were 12 percent more likely on April 20 after 4:20 p.m., the time the celebrations begin, than on the same day one week before or one week after, the study found.

Fallout from the festivities could be even deadlier for youth. Fatal crashes were 38 percent more likely for drivers under 21 years old after 4:20 p.m. on April 20 than they were the week before or after, Staples said.

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.

In Colorado, where marijuana is legal, Lyft, the ride-sharing service, last year launched a fleet of green cars wrapped with the message: “Plan a ride before you’re high.”

Jolene Forman, an attorney with Drug Policy Alliance, a New York nonprofit working to reduce the harms of both drug use and prohibition, said factors other than 4/20 celebrations might contribute to the increase in fatal crash rates. White supremacist groups, for example, also gather on April 20 to mark Adolf Hitler’s birthday.

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.

Legalization “provides an opportunity to shift our mindset away from prohibition and, instead, toward treating marijuana as a public health issue,” Forman said. “By treating marijuana as a public health issue, we can include marijuana in a comprehensive conversation about impaired and distracted driving, including the effects of prescription drugs, alcohol, texting, fatigue, etcetera on driving.”

Whitehill agreed that the study highlights numerous questions that remain to be answered about marijuana and driving.

“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.”

SOURCE: bit.ly/2EcM1zO JAMA Internal Medicine, online February 12, 2018.