THC-positive daytime drivers tripled after Washington State started retail cannabis sales | Research

The takeaway

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.

Cannabis users who drive, policymakers, and anyone interested in legalization impacts on road safety.

Daytime THC-positive drivers jumped from 8% to 23% after retail cannabis sales began

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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).

What this study cannot 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.

How to read the evidence

Moderate: biological testing with survey data across three timepoints, but cross-sectional design and one-year window.

When this study was published

Published in 2019, covering June 2014 to June 2015.

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.

Questions still open

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?

Common 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.

Read the original research

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

View on PubMed ↗Publisher’s paper ↗

Citation

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