California is running a controlled study of cannabis driving to measure how marijuana affects behind-the-wheel performance and to test law enforcement methods. The study enrolls about 300 participants and is the first major effort of its kind in nearly 40 years.
Researchers from the California Department of Motor Vehicles and the California Highway Patrol designed a driving course instrumented to record precise driver inputs. Instrumentation captures steering angle, pedal pressure, lane position, speed consistency and stopping behavior. Investigators use those objective measures alongside standardized field sobriety tests performed by CHP officers.
Participants undergo medical screening and are randomly assigned to one of three groups: cannabis with THC, placebo, or no cannabis. After smoking in a controlled booth, some people begin driving within minutes, while others wait up to an hour before starting. The University of California San Diego oversees the study’s human subjects review to ensure participant protections.
The DMV’s research chief, Bayliss Camp, noted that alcohol remains the leading substance in fatal and serious-injury crashes tied to impairment, and that cannabis ranks second. Capt. Elliotte Johnson, commander of the CHP Impaired Driving Section, highlighted a major change since prior research: average THC concentrations in retail cannabis have increased from roughly 1–4% in the 1970s–80s to commonly 20–30% or higher today.
Study specifics – Sample size: ~300 participants. – Groups: THC cannabis, placebo, no cannabis. – Timing: some drive within minutes after smoking; others after up to 60 minutes. – Vehicle data: steering inputs, pedal forces, lane crossings, stops and speed variability. – Law enforcement data: standardized field sobriety evaluations and Drug Recognition Expert assessments.
The DMV did not allow cameras during live testing to protect study integrity; ABC10 was permitted on-site but saw only CHP and DMV personnel, not study subjects. The study team intends to analyze how objective driving metrics change after cannabis consumption and whether current officer-observed indicators align with measurable driving impairment.
Law enforcement context CHP reports that specially trained Drug Recognition Experts conducted more than 8,000 suspected drug-impaired driver evaluations in 2025, with cannabis present in over 3,300 cases. Capt. Johnson said the agency is seeing more cannabis use across California and emphasized that impairment behind the wheel is preventable.
Research goals and timeline Researchers aim to identify which driving behaviors most reliably decline after cannabis use so training for officers can focus on observable, measurable signs. The study’s final report will go to DMV and CHP leadership and state officials before public release. Researchers expect to complete the study within roughly one year.
Why this matters now Cannabis products available today contain far higher THC concentrations than those used in earlier impairment research. That change in potency creates uncertainty about how quickly impairment appears after use and which behaviors safest correlate with reduced driving ability. By collecting high-resolution vehicle data and pairing it with field evaluations, the study will produce concrete measures—such as increased lane deviations per mile or greater steering variability—that policymakers and trainers can use to refine assessment protocols.
What the study will not do The research does not establish a single biological cutoff (like a blood alcohol concentration) that defines cannabis impairment. Officials say no equivalent roadside chemical test for cannabis impairment exists; officers must still rely on behavior, physical signs and standardized evaluations.
Next steps After internal review, the DMV and CHP will publish the study results. Agencies plan to use the report to adjust training materials, update officer testing procedures and inform public safety campaigns. The University of California San Diego will continue its human subjects oversight until the project closes and data release follows agency review.
Numbers to remember – 300 participants in the study – First major cannabis driving study in nearly 40 years – THC levels in retail products now commonly 20–30% or higher – 8,000+ drug recognition evaluations in 2025; cannabis present in >3,300 cases
The study seeks to replace uncertain assumptions with measurable comparisons of driving performance before and after cannabis use, and to test whether current field methods detect those changes reliably. Officials say the goal is clearer standards for identifying impaired drivers and reducing drug-related crashes on California roads.
