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.

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.

Study at a glance

Abstract

To investigate whether THC inhalation increases driving collisions, and to explore the relationship between car crashes and self-assessed driving ability as measured by Visual Analog Scales (VAS) and the Karolinska Sleepiness Scale, 30 male volunteers aged 18-30 years old, with a BMI of 18.5-26.2 Kg/m2, moderate tobacco and caffeine consumption, and at least one year of cannabis use either chronically (1-2 joints per day) or occasionally (1-2 joints per week) participated in a randomized, double-blind, crossover study comparing three doses of THC (placebo, 10 mg, and 30 mg). Participants received one cigarette with a different THC dose in three randomized sequences, separated by a seven-day washout period. Following each cigarette, participants were hospitalized for 24 h, during which blood samples were taken and driving performance was assessed using the York driving simulator. Additionally, self-assessment scales were completed to measure subjective feelings of awareness, calmness, confusion, adroitness, mental sharpness, and driving confidence. The number of car crashes on driving simulator increased from one hour, showing a significant increased and delayed effect observed 4 h post-inhalation of THC at 10 mg and 30 mg (Beta = 0.65, 95% CI = [0.45, 0.86], p < 0.001 and IRR = 1.9, p < 0.001), indicating this period as critical for assessing cannabis impairment and its implications for traffic safety. Although some crashes occur earlier, this delayed four hour-peak effect may suggests that the highest car crash risk on the simulator does not coincide with the period of maximum THC levels, which typically occur within 5 to 15 min of cigarette completion. Furthermore, blood THC-Cmax, was strongly associated with increased risk of car collisions at 4 h, (Beta = 3.63, 95% CI = [2.56, 4.70], p < 0.001 and IRR = 37.7, p < 0.001). The increase in sleepiness following cannabis consumption, measured by the Karolinska Sleepiness Scale, correlated with higher car crashes on simulator rates (Beta = 0.10, 95% CI = [0.05, 0.15], p < 0.001 and IRR = 1.1, p < 0.001). This study also found that driving collisions were positively associated with VAS adroitness (Beta = 0.010, 95% CI = [7.89e-03, 0.01], p < 0.001) and IRR = 1.0, p < 0.001. Remarkably, no significant difference in car collisions between occasional and chronic users was found, opposing the notion that chronic users are less impaired. Both chronic and occasional cannabis users have a similar car crash risk on simulator testing, with younger participants showing a higher risk, highlighting the need for targeted road safety campaigns. The direct link between subjective perceptions of driving proficiency and actual performance suggests that interventions aimed at improving self-awareness could be beneficial.