Understanding cannabis use and car crashes: Insights from a randomized trial using a driving simulator on THC blood levels and subjective measures of sleepiness and performance.
Journal of safety research December 1, 2025 Bibiana Cardozo, Sarah Hartley, Nicolas Simon et al.
In a randomized, double-blind, crossover study, 30 male cannabis users (aged 18–30) inhaled cigarettes with placebo, 10 mg, or 30 mg THC. Driving simulator crashes increased significantly, with a delayed peak at 4 hours post-inhalation, even though blood THC levels peak within 5–15 minutes. Higher blood THC concentrations and increased sleepiness (Karolinska Sleepiness Scale) were associated with more crashes. Surprisingly, self-assessed adroitness (VAS) correlated positively with crash rates, and no difference in crashes was found between chronic and occasional users, though younger participants had higher risk. The authors suggest this delayed effect period is critical for assessing cannabis impairment and road safety.