Effects of cannabis on driving can last up to 5 hours, warns international study
International study shows that the effects of cannabis on driving can last up to 5 hours, increasing the risk of accidents even when drivers are aware they are under the influence.
A study recently published in the Journal of Psychopharmacology revealed that the effects of cannabis on driving ability can extend for hours, even when users are aware they are under the influence. The research evaluated 38 adults aged 18 to 40, subjected to vaporized cannabis doses of 5.9% and 13% THC, as well as a placebo, in a randomized, double-blind, crossover study.
Detailed Methodology
Each participant underwent three days of eight-hour evaluations, with simulated driving tests conducted at four distinct times. Tasks such as lane maintenance, vehicle tracking, and overtaking were analyzed, totaling 19 behavioral metrics. The statistical analysis used mixed linear models in SPSS, assessing the main effects of dose, time, and dose × time interaction.
Main Findings
Lane Maintenance: participants showed a reduction in the rate of directional reversal for up to 5.5 hours after the 13% THC dose and 3.5 hours with 5.9%.
Vehicle Tracking: changes in pedal use, including reduced range of motion and lower correction rates, persisted for 1 to 3 hours, only with the higher dose.
Overtaking: after 13% THC, drivers maintained a shorter distance from overtaken vehicles, had a shorter time to potential collision, and spent more time in the oncoming lane, increasing the risk of accidents.
Perception vs. Actual Performance
Despite being aware of the drug's effects, about 66% of participants stated they would be willing to drive, revealing a concerning discrepancy between subjective perception and actual performance behind the wheel. Furthermore, the study found no significant correlation between THC metabolite concentration in the blood and driving performance, questioning the reliability of blood tests as the sole risk indicator.
Implications for Road Safety
The authors emphasize that the prolonged effects of cannabis require broader approaches to risk assessment for driving, which go beyond simple measurement of THC in the blood. The study also broadens the attention window on driving under the influence of the drug, showing that impacts can last longer than the three hours typically explored in previous research.