The April 20 Cannabis Celebration and Fatal Traffic Crashes in the United States

Abstract

This data analysis examines the association between the April 20 cannabis celebrations in the United States and a population-level increase in the risk of fatal traffic crash involvement.

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. In many cities, activists and enthusiasts gather at public celebrations that feature synchronized mass consumption of cannabis at 4:20 pm.

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. Despite this evidence, driving after cannabis consumption is surprisingly common. 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.

Figure 1. Relative Risk of Traffic Crash on April 20.

Forest plot showing relative increase in risk of a traffic crash on April 20 compared with control days exactly 1 week earlier and later. Solid squares indicate point estimate; relative dimensions, sample size; and horizontal lines, 95% CIs. Vertical columns show total counts between 4:20 PM and 11:59 PM on April 20 and control days. Main findings show an increase in relative risk on April 20, no significant contrary findings, and an accentuated effect for younger individuals. State-level estimates of the prevalence of marijuana use among adults from the 2002-2003 National Surveys on Drug Use and Health (near the midpoint of our study) were used to categorize states as "Higher use" (median and above) and "Less high use" (below the median).

Figure 2. National Variation in Relative Risk of Traffic Crash On April 20.

The map displays the United States for all contiguous states and offset images for noncontiguous states. Colors denote relative increase in traffic risks on April 20 compared with control days exactly 1 week earlier and later. Green corresponds to increased relative risk, yellow to neutral relative risk, and brown to decreased relative risk (spectrum scaled by logarithm transformation). Findings show that 30 states had a relative risk point estimate greater than unity (eg, Hawaii), while only 18 states had an estimate less than unity (eg, Minnesota), and 2 states had an estimate of exactly unity (eg, Montana).

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. Policy makers may wish to consider these risks when liberalizing marijuana laws, paying particular attention to regulatory and enforcement strategies to curtail drugged driving.