THC-positive daytime drivers tripled after Washington State started retail cannabis sales
Roadside surveys in Washington State found daytime THC-positive drivers jumped from 8% to 23% after recreational retail sales began, while self-reported use rates stayed flat, revealing a gap between what drivers say and what tests show.
Quick Facts
What This Study Found
THC-positive daytime drivers increased from 8% before retail sales to 23% six months after. Nighttime THC prevalence was unchanged (19-20%). Drivers who perceived marijuana as very likely to impair driving had 40% lower odds of testing THC-positive. Self-reported use did not predict changes.
Key Numbers
2,355 drivers surveyed across 3 waves. Daytime THC-positive: 8% pre-retail to 23% post-retail. Nighttime THC-positive: 19% to 20% (unchanged). Perceived impairment risk reduced THC-positive odds by 40%. Perceived arrest risk had no effect.
How They Did This
Three-wave roadside survey of 2,355 drivers in Washington State (June 2014, November-December 2014, June 2015) with marijuana questionnaires and biological specimens (oral fluid, blood for THC, breath for alcohol).
Why This Research Matters
This is one of the first studies to combine roadside biological testing with survey data around a legalization event. The disconnect between self-reported use and test results suggests surveys alone underestimate actual cannabis-impaired driving prevalence.
The Bigger Picture
Unlike alcohol, where impaired driving peaks at night, cannabis-impaired driving after legalization was equally prevalent day and night. This challenges conventional enforcement strategies that concentrate resources on nighttime checks.
What This Study Doesn't Tell Us
THC-positive status does not necessarily indicate impairment at the time of driving. One year of follow-up may not capture long-term trends. Washington State results may not generalize to other states with different markets and demographics.
Questions This Raises
- ?Does the daytime increase reflect more use or just more willingness to drive after use?
- ?Has cannabis-impaired driving in Washington continued to increase beyond the one-year window studied?
- ?Would public education campaigns about impairment risk reduce THC-positive driving?
Trust & Context
- Key Stat:
- Daytime THC-positive drivers jumped from 8% to 23% after retail cannabis sales began
- Evidence Grade:
- Moderate: biological testing with survey data across three timepoints, but cross-sectional design and one-year window.
- Study Age:
- Published in 2019, covering June 2014 to June 2015.
- Original Title:
- Marijuana use and driving in Washington State: Risk perceptions and behaviors before and after implementation of retail sales.
- Published In:
- Traffic injury prevention, 20(1), 23-29 (2019)
- Authors:
- Eichelberger, Angela H(2)
- Database ID:
- RTHC-02021
Evidence Hierarchy
A snapshot of a population at one point in time.
What do these levels mean? →Frequently Asked Questions
Did legalization increase cannabis-impaired driving?
In Washington State, daytime THC-positive drivers nearly tripled from 8% to 23% within months of retail sales opening. However, THC-positive does not necessarily mean impaired at the time of testing.
Why didn't nighttime THC-positive rates change?
Nighttime rates were already around 19-20% before retail sales and stayed there. The study suggests cannabis use while driving was already common at night; legalization mainly shifted daytime patterns.
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Cite This Study
https://rethinkthc.com/research/RTHC-02021APA
Eichelberger, Angela H. (2019). Marijuana use and driving in Washington State: Risk perceptions and behaviors before and after implementation of retail sales.. Traffic injury prevention, 20(1), 23-29. https://doi.org/10.1080/15389588.2018.1530769
MLA
Eichelberger, Angela H. "Marijuana use and driving in Washington State: Risk perceptions and behaviors before and after implementation of retail sales.." Traffic injury prevention, 2019. https://doi.org/10.1080/15389588.2018.1530769
RethinkTHC
RethinkTHC Research Database. "Marijuana use and driving in Washington State: Risk percepti..." RTHC-02021. Retrieved from https://rethinkthc.com/research/eichelberger-2019-marijuana-use-and-driving
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.