Marijuana, alcohol and actual driving performance

Ramaekers, Johannes G.; Robbe, Hindrik W.J.; John F. O’Hanlon · 2000 · OpenAlex-citations

DOI: 10.1002/1099-1077(200010)15:7<551::aid-hup236>3.0.co;2-p

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Summary

This study empirically determined the separate and combined effects of Delta-9-tetrahydrocannabinol (THC) and alcohol on actual driving performance. It was motivated by conflicting evidence from previous research: epidemiological data suggested a synergistic interaction between THC and alcohol that increased crash risk, while prior experimental studies using driving simulators or closed courses failed to confirm this synergy, often citing methodological shortcomings and a lack of ecological validity. This research aimed to resolve this discrepancy by testing driving performance in a natural environment—on real roads in normal traffic. The study employed a balanced, six-way, observer- and subject-blind, cross-over design with 18 licensed drivers (aged 20–28) who used both substances regularly. Participants were administered weight-calibrated doses of THC (100 µg/kg and 200 µg/kg via smoking) and alcohol (targeting a peak blood alcohol concentration [BAC] of 0.07 g/dl, sustained at 0.04 g/dl during testing), or placebos, across separate evenings. Thirty minutes after smoking, subjects performed two standardized driving tests: a Road Tracking Test, measuring the ability to maintain a constant speed of 100 km/h and steady lateral position, and a Car Following Test, measuring reaction times and headway variability while following a vehicle executing acceleration and deceleration maneuvers. Performance was objectively measured via standard deviation of lateral position (SDLP), time out of lane (TOL), and reaction time, alongside subjective ratings from subjects and licensed driving instructors. The results demonstrated that both THC and alcohol, administered alone, significantly impaired driving performance. The magnitude of impairment was minor for alcohol alone and for the lower THC dose (100 µg/kg), but moderate for the higher THC dose (200 µg/kg). However, the combination of THC with alcohol severely impaired performance in both tests. Specifically, the combination of alcohol and 100 µg/kg THC produced a rise in SDLP equivalent to a BAC of 0.09 g/dl, while the combination with 200 µg/kg THC was equivalent to a BAC of 0.14 g/dl. Time out of lane increased exponentially with drug effects, reaching 1.1% under the combined high-dose condition. Reaction times lengthened by 36% and headway variability increased by 37% under the combined influence of alcohol and 200 µg/kg THC. Instructors noted that aberrant behaviors observed in combined drug conditions would have been dangerous in natural driving scenarios. The study concludes that while THC alone causes dose-related impairment that persists for hours, the combination of THC and alcohol produces severe deficits in driving ability comparable to those seen at BAC levels above the legal intoxication limit of 0.08 g/dl in some jurisdictions. Although the study did not find unequivocal evidence of a classic pharmacological synergy, the exponential increase in lane deviation suggests that the interaction significantly increases crash risk. The findings imply that the practical consequences of driving under the combined influence of marijuana and alcohol are severe, regardless of whether the pharmacological interaction is strictly synergistic or additive.

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Topics

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• polydrug
• cannabis
• alcohol
• illicit drugs