Marijuana and Actual Driving Performance

Robbe, Hindrik W.J.; James F. O’Hanlon · 1993 · OpenAlex-citations

DOI: 10.1037/e730492011-001

archive: archived pipeline: cataloged verified

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Summary

This report investigates the effects of marijuana smoking on actual driving performance, addressing the gap between epidemiological data and controlled experimental evidence regarding traffic safety. While epidemiological studies link marijuana use to accidents, they are often confounded by alcohol co-use. Previous simulator studies suggested minor impairment, but this research aimed to determine the dose-response relationship between delta-9-tetrahydrocannabinol (THC) and real-world driving, and to assess whether plasma drug concentrations could predict impairment. The study was conducted by the University of Limburg for the U.S. National Highway Traffic Safety Administration. The research program comprised one pilot study and three actual driving studies involving recreational cannabis users. The pilot study established that users typically consume approximately 300 µg/kg of THC to achieve their desired effect, setting this as the maximum dose for subsequent trials. The first driving study tested doses of 0, 100, 200, and 300 µg/kg on a closed highway, measuring lateral position variability. The second study replicated this on a public highway with other traffic, adding a car-following test to assess reaction times and headway maintenance. The third study compared a modest THC dose (100 µg/kg) against a low alcohol dose (0.04 g% BAC) during urban city driving, using both observer ratings and specific behavioral metrics. All subjects were monitored by licensed instructors with redundant vehicle controls for safety. The findings indicate that marijuana produces a moderate, dose-related impairment in driving performance, primarily manifested as an increased standard deviation of lateral position (SDLP). This impairment persisted for nearly two hours after smoking, even as plasma THC levels and subjective feelings of intoxication declined. The magnitude of impairment from 300 µg/kg THC was comparable to that of blood alcohol concentrations between 0.03 and 0.07 g%. In car-following tests, subjects increased their headway and reaction times, particularly at lower doses, suggesting heightened caution. Crucially, plasma concentrations of THC or its metabolites did not correlate with driving impairment. In the urban study, alcohol significantly impaired driving performance, whereas marijuana did not significantly alter objective scores; however, marijuana users reported higher effort and perceived their performance as impaired, unlike alcohol users who remained unaware of their deficits. The significance of these results lies in the conclusion that marijuana causes measurable but moderate driving impairment that is not predictable by blood tests or standard roadside sobriety assessments. Unlike alcohol, which often leads to overconfidence, marijuana users retain insight into their condition and compensate by increasing effort and caution, such as slowing down. Consequently, while THC affects vehicle control, the overall risk to traffic safety appears relatively small compared to alcohol, provided drivers are aware of their state. The study highlights the limitations of using plasma drug levels to assess fitness to drive and underscores the importance of subjective awareness in mitigating impairment.

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Topics

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• cannabis
• polydrug