In a 2010 California roadside survey, 8.5% of weekend nighttime drivers tested positive for THC, with rates as high as 18.3% in some jurisdictions, and THC-positive rates had increased since 2007.
Read this if you want to understand how common cannabis-involved driving is.
8.5% of weekend nighttime drivers tested positive for THC
What the researchers found
Researchers collected anonymous oral fluid samples and breath tests from over 900 weekend nighttime drivers across six California jurisdictions. Overall, 14.4% tested positive for illegal drugs, with 8.5% positive for THC specifically.
THC-positive rates varied enormously by location: from 4.3% in Fresno to 18.3% in Eureka (a cannabis-producing region). Compared to 2007 National Roadside Survey data, THC-positive rates had increased, while rates for other illegal drugs had not.
Drivers with medical cannabis permits were significantly more likely to test positive for THC, suggesting medical cannabis legislation contributed to increased cannabis-involved driving.
Why it matters
Nearly 1 in 10 weekend nighttime drivers testing positive for THC represented a significant road safety concern. The geographic variation and increase over time pointed to the influence of cannabis policies on driving behavior.
The numbers in context
900+ drivers sampled. 14.4% positive for illegal drugs. 8.5% positive for THC. Range: 4.3% (Fresno) to 18.3% (Eureka). Medical cannabis permit holders: significantly more likely THC-positive. THC rates increased from 2007 to 2010.
How the study worked
Cross-sectional roadside survey. Anonymous oral fluid and breath samples from 900+ weekend nighttime drivers across six California jurisdictions. Compared to 2007 National Roadside Survey data collected with comparable methods.
What this study cannot tell us
THC-positive oral fluid does not necessarily indicate impairment at the time of testing. Weekend nighttime drivers may not represent all driving. Anonymous design prevented following up on driving outcomes. No crash data were linked.
How to read the evidence
Well-designed roadside survey with biological testing. Strong prevalence data but cannot determine impairment or crash risk from THC-positive oral fluid.
When this study was published
Published in 2012. California legalized recreational cannabis in 2016. THC-involved driving rates have been studied extensively since.
The bigger picture
As cannabis legalization expanded, understanding its impact on road safety became critical. This study provided baseline data showing THC-involved driving was already common and increasing, creating urgency for impaired driving countermeasures.
Questions still open
- What proportion of THC-positive drivers were actually impaired? Has recreational legalization further increased THC-involved driving? Are medical cannabis patients at higher crash risk? What THC concentration indicates impairment?
Common questions
How many drivers are using cannabis?
Does testing positive mean the driver was impaired?
Read the original research
The prevalence of cannabis-involved driving in California.
Drug and alcohol dependence, 123(1-3), 105-9
Citation
Johnson, Mark B; Kelley-Baker, Tara; Voas, Robert B; Lacey, John H. (2012). The prevalence of cannabis-involved driving in California.. Drug and alcohol dependence, 123(1-3), 105-9. https://doi.org/10.1016/j.drugalcdep.2011.10.023