On April 20 each year, thousands of Americans celebrate the intoxicating properties of marijuana on a popular counterculture holiday known as “4/20.” Legal marijuana sales surge in anticipation of the “High Holiday,” and college students report increased cannabis consumption on 4/20 itself.1,2 In many cities, activists and enthusiasts gather at public celebrations that feature synchronized mass consumption of cannabis at 4:20 pm.3

Driving simulation studies indicate that higher blood Δ9-tetrahydrocannabinol concentrations decrease reaction times and increase variability in speed and lane position, while some epidemiologic studies suggest that acute cannabis intoxication increases crash risk.4 Despite this evidence, driving after cannabis consumption is surprisingly common.5 We hypothesized that the April 20 cannabis celebration might be associated with a population-level increase in the risk of fatal traffic crash involvement.

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 (ie, 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.

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Corresponding Author: John A. Staples, MD, MPH, St Paul’s Hospital, 1081 Burrard St, Burrard Bldg, Rm 5910, Vancouver, BC V6Z 1Y6, Canada (jstaples1@providencehealth.bc.ca).

Accepted for Publication: December 2, 2017.

Published Online: February 12, 2018. doi:10.1001/jamainternmed.2017.8298

Author Contributions: Dr Staples had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: All authors.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: All authors.

Critical revision of the manuscript for important intellectual content: All authors.

Statistical analysis: All authors.

Administrative, technical, or material support: All authors.

Study supervision: Redelmeier.

Conflict of Interest Disclosures: None reported.

Funding/Support:This work was supported by the Vancouver Coastal Health Research Institute, the Canadian Institutes of Health Research, and the Canada Research Chair in Medical Decision Science.

Role of the Funder/Sponsor: The funders/sponsors had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Additional Contributions: We thank Clinton Pecenka, MPP, PhD, and Mark Staples, PhD, (Massachusetts Institute of Technology, Cambridge) for insightful comments on an earlier draft of this work. No compensation was provided for their contributions.