Increase in fatal traffic crashes on annual US cannabis celebration

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Car Crash with policeDrivers in the US are more likely to be in a fatal traffic crash during the annual 20 April cannabis celebration, according to research from the University of British Columbia. “One-fifth of Americans now live in states that have legalised recreational cannabis, and legalisation is set to occur for all Canadians in July 2018,” said lead researcher Dr John Staples, a clinical assistant professor of medicine and scientist at University of British Columbia’s Centre for Health Evaluation and Outcome Sciences. “We hope that legalisation doesn’t lead to more people driving while high.”

Along with University of Toronto professor Dr Donald Redelmeier, Staples examined 25 years of National Highway Traffic Safety Administration data on all fatal traffic crashes in the US. They compared the number of drivers involved in fatal crashes after 4:20 p.m. on 20 April with the number of drivers involved in fatal crashes during the same time intervals on control days one week earlier and one week later.

The investigators found that 20 April was associated with a 12% increase in the risk of a fatal traffic crash. Among drivers younger than 21 years of age, the risk was 38% higher than on control days. The overall increase amounted to 142 additional deaths over the 25-year study period.

Since the 4/20 holiday was first popularised in 1991, annual events in Denver, San Francisco, Vancouver, and other cities have grown to include tens of thousands of attendees. It isn’t known how commonly drivers get behind the wheel while high on 4/20, but a 2011 study of US college freshmen found 44% of cannabis users drove soon after consuming marijuana in the month prior to the survey. Only half of cannabis users in the 2017 Canadian Cannabis Survey thought cannabis use affected driving.

“Assuming fewer than 10% of Americans drive while high on 20 April, our results suggest that drug use at 4/20 celebrations more than doubles the risk of a fatal crash,” said Redelmeier.

Staples and Redelmeier hope that authorities will respond to these results by encouraging safer 4/20 travel options, including public transit, rideshares, taxis and designated drivers. The investigators also note that cannabis retailers and 4/20 event organizers have an opportunity to serve their customers and save lives by warning users not to drive while high.

As Canada and other places move toward legalisation, Staples says it’s also important to employ multiple strategies to reduce driving under the influence of drugs throughout the year. “Driving is a potentially dangerous activity,” Staples said. “Improving road safety requires both policymakers and drivers to make smart decisions. If you’re going to get behind the wheel, buckle up, put the phone away, don’t speed, stay sober and don’t drive high.”

The research was supported by Vancouver Coastal Health Research Institute, the Canadian Institutes for Health Research, and the Canada Research chair in medical decision science.

Methods: This study used publicly available statistical data with a waiver of approval from the University of British Columbia research ethics board. We obtained fatal motor vehicle crash data from the United States National Highway Traffic Safety Administration’s Fatality Analysis Reporting System (FARS). FARS includes data on all crashes involving a motor vehicle traveling on public roadways in which at least 1 participant died within 30 days of the event. The study interval began the first full year after popularization of 4/20 in High Times magazine and extended to include the most recent year with data available, thereby reflecting 25 consecutive years (January 1992 to December 2016).
The primary analysis compared the number of drivers involved in fatal traffic crashes between 4:20 PM and 11:59 PM on April 20 each year to the number in fatal traffic crashes during the same time intervals on control days 1 week earlier and 1 week later (April 13 and April 27). This design controlled for weekday, season, and year while minimizing bias from changes in vehicle design, travel distances, medical care and other confounders. We tested for differences between case and control dates using exact binomial tests and performed prespecified stratified analyses to examine individual factors and geographic variation.
Results: The 25-year study interval identified 1.3 million drivers involved in 882 483 crashes causing 978 328 fatalities. In total, 1369 drivers were involved in fatal crashes after 4:20 PM on April 20 whereas 2453 drivers were in fatal crashes on control days during the same time intervals (corresponding to 7.1 and 6.4 drivers in fatal crashes per hour, respectively). The risk of a fatal crash was significantly higher on April 20 (relative risk, 1.12; 95% CI, 1.05-1.19; P = .001).
Subgroup analysis suggested the relative risk increase on April 20 was particularly pronounced for younger drivers (Figure 1). No subgroup suggested April 20 conferred a contrary protective effect. Geographic analysis suggested the absolute risk increases were greatest in New York (excess of 36), Texas (excess of 32), and Georgia (excess of 29). Relative risk was significantly reduced only for Minnesota (Figure 2). Sensitivity analyses supported the results of the primary analysis, including evaluations of all crash-involved persons (rather than only drivers) and extended time intervals (4:20 PM to 5:00 AM the following day). As expected, the number of drivers involved in crashes earlier than 4:20 PM or on nearby dates (April 18 and April 22) was no different than corresponding controls.
Discussion: We examined a quarter-century of national data and found a 12% increase in the relative risk of a fatal traffic crash after 4:20 PM on April 20 compared with identical time intervals on 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.6 Policy makers may wish to consider these risks when liberalizing marijuana laws, paying particular attention to regulatory and enforcement strategies to curtail drugged driving.

John A Staples, Donald A Redelmeier

University of British Columbia material
JAMA Internal Medicine abstract

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