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This article was published 7/3/2019 (225 days ago), so information in it may no longer be current.
A pair of Canadian researchers earned international headlines last year with a study that found fatal car crashes in the U.S. were much more likely after 4:20 p.m. on April 20, the counter-culture cannabis holiday, but new research that replicates and expands on the 2018 study casts doubt on whether its provocative finding is accurate.
The original study, published in February 2018 as a research letter in the prestigious scientific journal JAMA Internal Medicine, was co-authored by Canadian physicians John Staples and Donald Redelmeier. The authors tested their hypothesis that a presumed spike in the number of people using cannabis on April 20 might increase the risk of fatal car accidents with a statistical analysis of in-depth traffic safety data from a U.S. government database.
The original methodology was straightforward. The authors collected 25 years worth of data on the number of drivers in fatal car crashes between 4:20 p.m. (when 4/20 revellers presumably light up) and 11:59 p.m. on April 20. Then, they compared that figure to two control days — one a week before April 20, and one a week after.
All told, the authors concluded the relative risk of a fatal car accident was about 12 per cent higher on April 20 than at the same time on the control days.
"Although the vast majority of Americans do not celebrate 4/20, the observed association was comparable in magnitude to the increase in traffic risks observed on Superbowl Sunday," wrote Staples and Redelmeier.
But a more recent, in-depth analysis of the same data by two McGill University researchers reaches a different conclusion.Advertisement
Lead author Sam Harper, an associate professor of epidemiology at McGill, said he was intrigued by the original finding, but had some doubts.
"I think my initial reaction was probably skepticism about the magnitude of the impact," said Harper in an interview.
"I mean, the idea that 4/20 celebrations could raise the national rate of fatal car crashes by something like 12 per cent, I found to be a little bit implausible."
Harper and co-author Adam Palayew published their peer-reviewed paper on the topic this past January in the journal Injury Prevention. Using the same U.S. traffic safety database as the authors of the original study, Harper and Palayew started by replicating the original statistical analysis. They found the same result: a 12 per cent increase in fatal crashes on April 20 compared to the two control days dated a week before and after.
Then, they tested the 4/20 fatal crash risk data against the data for other control days. First, they examined how the number of fatal crashes on April 20 compared to the number of fatal crashes on four control days, one week and two weeks before and after April 20. Then they repeated the same analysis using every other day of the year.
"When you look at either four days on either side, or you look at all other days, you really don't see any impact at all of 4/20," said Harper.
Harper and Palayew also compared the odds of a fatal crash on April 20 to the same odds on days with a well-documented increase in deadly road accidents in the U.S.: Independence Day, Thanksgiving and Labour Day.
"These are other places where you see really reliable signals of elevated fatal car crashes, and that's because these are tied, primarily, to drunk driving," said Harper.
Compared to those known deadly days on American roads, Harper's analysis found the odds of a fatal car crash on April 20 look a lot like the odds on any other day of the year.
Still, Harper stopped short of claiming his paper debunks the 2018 study that received so much attention; instead, he sees it as building upon the original study as part of the slow, painstaking process of public health research. (He also took pains to note his study does not mean it's safe to drive after using cannabis.)
Unlike the 2018 study published in JAMA Internal Medicine, though, Harper said his paper has received very little media attention. (In fact, the first inquiry he said he had received was from The Leaf News.)
"And I will say, I do find this, as an academic, sort of a frustrating part of the research-to-policy-to-public cycle that we go through," he said.
"There's a lot of emphasis in the publishing world, I think, on trying to find sexy, novel findings… There's also a lot of appetite for scaremongering, which is a little frustrating."
Dr. John Staples, the lead author of the 2018 study, declined a request for an interview. In an email, he said he and his co-author are aware of Harper's analysis and are "considering these points and doing some analysis of our own in response."
Updated on Thursday, March 7, 2019 at 4:05 PM CST: Corrects title for Sam Harper to "associate professor".