No evidence that April 20th "cannabis holiday" increases fatal traffic crashes nationally
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.
Quick Facts
What This Study 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).
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- April 20th fatal crash rate: 0.98 relative to all other days (no increase)
- Evidence Grade:
- Strong: 42 years of national data with multiple control period comparisons addressing prior methodological limitations.
- Study Age:
- Published in 2019, using 1975-2016 data.
- Original Title:
- The annual cannabis holiday and fatal traffic crashes.
- Published In:
- Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention, 25(5), 433-437 (2019)
- Authors:
- Harper, Sam(2), Palayew, Adam
- Database ID:
- RTHC-02064
Evidence Hierarchy
Looks back at existing records to find patterns.
What do these levels mean? →Frequently Asked Questions
Does 4/20 cause more car crashes?
This analysis of 42 years of data found no meaningful increase. A prior study claimed a 12% increase, but that depended on which comparison days were chosen. With broader control periods, the effect disappeared entirely.
Why did the original 4/20 study find an effect?
It compared April 20th only to days one week before and after. This analysis shows the supposed effect vanishes when compared to all other days of the year, suggesting the original finding was a statistical artifact of narrow control selection.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-02064APA
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
MLA
Harper, Sam, et al. "The annual cannabis holiday and fatal traffic crashes.." Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention, 2019. https://doi.org/10.1136/injuryprev-2018-043068
RethinkTHC
RethinkTHC Research Database. "The annual cannabis holiday and fatal traffic crashes." RTHC-02064. Retrieved from https://rethinkthc.com/research/harper-2019-the-annual-cannabis-holiday
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.