Analysis of 42 years of US fatal crash data found little evidence that the annual April 20th cannabis celebration causes a population-wide increase in drivers involved in fatal traffic crashes.
People interested in cannabis and driving safety, researchers evaluating event-study methodology, and anyone who saw the original 4/20 crash study headlines.
April 20th fatal crash rate: 0.98 relative to all other days (no increase)
What the researchers found
Between 1992 and 2016, April 20th showed a non-significant 12% increase in fatal crash involvement relative to control days one week apart (IRR 1.12, 95% CI: 0.97-1.28), but no increase when compared to broader control periods or all other days (IRR 0.98, 95% CI: 0.88-1.10).
Why it matters
A prior widely-cited study claimed 4/20 increased fatal crashes by 12%. This more thorough analysis shows that finding depends entirely on which control days are chosen, and the effect disappears with broader comparisons.
The numbers in context
IRR vs 1-week controls: 1.12 (95% CI: 0.97-1.28, not significant). IRR vs 1-2 week controls: 1.05 (95% CI: 0.92-1.28). IRR vs all other days: 0.98 (95% CI: 0.88-1.10). Data spans 1975-2016.
How the study worked
Analysis of Fatal Analysis Reporting System data from 1975-2016, comparing drivers involved in fatal crashes on April 20th (16:20-23:59) against multiple control periods including one week, two weeks, and all other days of the year.
What this study cannot tell us
Cannot distinguish cannabis-specific effects from general holiday or celebration effects. Cannabis use on 4/20 is not measured directly. The analysis assumes fatal crash involvement reflects impairment risk, which depends on many factors.
How to read the evidence
Strong: 42 years of national data with multiple control period comparisons addressing prior methodological limitations.
When this study was published
Published in 2019, using 1975-2016 data.
The bigger picture
This is an example of how methodological choices can inflate apparent effects. The cannabis holiday study is a cautionary tale about cherry-picking control periods to generate headline-worthy findings that don't hold up under more rigorous analysis.
Questions still open
- Do other cannabis-related events or policy changes show clearer effects on traffic safety? Could local-level analysis reveal effects masked in national data? How should researchers select appropriate control periods for event studies?
Common questions
Does 4/20 cause more car crashes?
Why did the original 4/20 study find an effect?
Read the original research
The annual cannabis holiday and fatal traffic crashes.
Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention, 25(5), 433-437
Citation
Harper, Sam; Palayew, Adam. (2019). The annual cannabis holiday and fatal traffic crashes.. Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention, 25(5), 433-437. https://doi.org/10.1136/injuryprev-2018-043068