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In this April 20, 2016, file photo, people smoke marijuana during a 4/20 rally in Toronto.

Mark Blinch/AP

Drivers across the United States are significantly more likely to be in a fatal car crash in the hours immediately after annual 4/20 cannabis celebrations, according to a new UBC study of 25 years of official data.

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.

John Staples, a professor of medicine at the University of British Columbia, and Donald Redelmeier, of the University of Toronto, compared the number of drivers involved in deadly crashes from 4:20 p.m. to midnight that day – long celebrated by stoner culture – to the same time span on a day a week earlier and a day a week later. The overall increase led to 142 more deaths during that quarter-century, Dr. Staples said.

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"The big surprise for me was the magnitude of the effect," he told The Globe and Mail. "The other thing that was very striking to me was the size of the effect that we saw in younger individuals."

Among those 21 and under who got behind a wheel, there was a 38-per-cent greater risk of a fatal accident during the second half of April 20, according to the study.

"In general, we know that younger drivers are vulnerable drivers and that probably arises from a lack of experience and maybe a tendency to take a few more risks," said Dr. Staples. "They might be particularly sensitive to intoxicants like cannabis and other elements of 4/20 that might be unusual.

"That might be an indication of where we should focus our efforts," he said of younger people driving high.

Ensuring there is no post-legalization spike in drug-impaired driving is a core concern for the federal Liberal government as it continues to work toward completing this historic policy shift this summer.

A number of high-profile senators now debating legalization have that cited data from Colorado and Washington is behind their concern over Ottawa's strategy to combat a potential rise in the number of drug-impaired drivers.

In 2013, there were 18 such fatal incidents in Colorado; by 2016, that number had risen to 77. In Washington state, the number of drivers involved in fatal crashes that tested positive for THC – a psychoactive compound in cannabis – averaged 33 a year between 2008 and 2013; since then, there has been an average of 78 such drivers. Both states started legalized cannabis sales in 2014.

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Senators are worried about the lack of drug-recognition experts (DREs) that will be trained to detect and charge impaired drivers after their oral-fluid samples test positive in Canada. Under C-46, a conviction for drug-impaired driving will require a blood test or a finding by a DRE, and not just a positive oral-fluid test.

Appearing in front of a Senate committee two weeks ago, Public Safety Minister Ralph Goodale said there are currently 550 DREs in Canada, with plans to double that number in coming years. However, the Canadian Centre on Substance Abuse estimated in 2009 that up to 2,000 DREs would be needed to meet demand, which would require current numbers to quadruple. In 2008, the Canadian Association of Chiefs of Police estimated that Canada would need 3,000 DREs to meet international standards.

Dr. Staples said he got the idea for his study while seeing people intoxicated from downtown Vancouver's annual massive 4/20 party stream into the emergency room of St. Paul's Hospital, where he works as a general internist.

Jeff Brubacher, a Vancouver ER doctor and colleague of Dr. Staples, published research three years ago that surveyed just over 1,000 drivers injured across B.C. and found cannabis was the most common recreational drug, after alcohol, used among them, with 7.3 per cent consuming it in the hours preceding their crashes and 12.6 per cent still showing traces of the drug from earlier use.

Cannabis users should wait at least four hours before getting behind the wheel, Dr. Brubacher has recommended, in order to counteract the way the drug distracts people, slows reaction times, causes weaving and makes it harder to maintain a constant speed.

Federal MP Bill Blair says studying the effects of legalized recreational marijuana will help to understand the impact of the country's new pot laws. The government announced on Wednesday $1.4 million for research into cannabis. The Canadian Press
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