Nearly 1 in 10 weekend nighttime drivers in California tested positive for THC
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- 8.5% of weekend nighttime drivers tested positive for THC
- Evidence Grade:
- Well-designed roadside survey with biological testing. Strong prevalence data but cannot determine impairment or crash risk from THC-positive oral fluid.
- Study Age:
- Published in 2012. California legalized recreational cannabis in 2016. THC-involved driving rates have been studied extensively since.
- Original Title:
- The prevalence of cannabis-involved driving in California.
- Published In:
- Drug and alcohol dependence, 123(1-3), 105-9 (2012)
- Authors:
- Johnson, Mark B(2), Kelley-Baker, Tara(3), Voas, Robert B(3), Lacey, John H
- Database ID:
- RTHC-00576
Evidence Hierarchy
A snapshot of a population at one point in time.
What do these levels mean? →Frequently Asked Questions
How many drivers are using cannabis?
In this 2010 California survey, 8.5% of weekend nighttime drivers tested positive for THC, meaning roughly 1 in 12 drivers on the road at night had recently used cannabis. In some areas (Eureka), it was nearly 1 in 5.
Does testing positive mean the driver was impaired?
Not necessarily. THC can be detected in oral fluid after the impairing effects have worn off, especially in regular users. However, the high prevalence of THC-positive drivers indicates widespread cannabis-involved driving that warrants public safety attention.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-00576APA
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
MLA
Johnson, Mark B, et al. "The prevalence of cannabis-involved driving in California.." Drug and alcohol dependence, 2012. https://doi.org/10.1016/j.drugalcdep.2011.10.023
RethinkTHC
RethinkTHC Research Database. "The prevalence of cannabis-involved driving in California." RTHC-00576. Retrieved from https://rethinkthc.com/research/johnson-2012-the-prevalence-of-cannabisinvolved
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.