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The Effects of Cannabis on Driving Skills

Thursday, October 11, 2012 — Poster Session III

10:00 a.m. – Noon

Natcher Conference Center, Building 45




  • R.L. Hartman
  • M.A. Huestis


Background: Cannabis is the most prevalent illicit drug identified in impaired drivers. Cannabis’ effects on driving continue to be debated, making prosecution and legislation difficult Objective: Review and evaluate current knowledge of cannabis’ effects on driving. Methods: Electronic databases were searched with no time restriction for keywords ‘cannabis’, ‘marijuana’, ‘automobile driving’, ‘accidents, traffic’, ‘motor vehicles’. Additional articles selected from references in identified sources. Results: Epidemiology: Risk of motor vehicle accident (MVA) involvement significantly increased, approximately twofold, with cannabis. Experimental: Drivers with cannabis attempted to compensate by driving more slowly, but control deteriorated with increasing task complexity. Cannabis increased lane position variability, and impaired cognitive function. There was evidence for tolerance in heavy smokers on critical tracking and divided attention tasks. Combining cannabis with alcohol enhanced impairment, especially lane weaving. Conclusion: Different study designs frequently accounted for result inconsistencies. Selection bias and confounding factors attenuated ostensible cannabis effects, but association with MVAs often retained significance. Recent smoking and/or blood THC concentrations ≥2 to 5 ng/mL (task-dependent) are associated with significant driving impairment, particularly in occasional smokers. Future cannabis-driving research should emphasize challenging tasks, such as divided attention, and include occasional and chronic daily cannabis smokers. Funded by the IRP, NIDA, NIH

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