Marijuana Smoking Associated With Minimal Changes In Driving Performance, Study Finds

{njaccess 0,18}Hartford, CT—(ENEWSPF)—May 28, 2010 Subjects exhibit virtually identical psychomotor skills on a battery of driving simulator tests prior to and shortly after smoking marijuana, according to clinical trial data published in the March issue of the Journal of Psychoactive Drugs.

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Investigators from Hartford Hospital in Connecticut and the University of Iowa Carver College of Medicine assessed the simulated driving performance of 85 subjects in a double-blind, placebo controlled trial. Volunteers responded to various simulated events associated with automobile crash risk — such as avoiding a driver who was entering an intersection illegally, deciding to stop or go through a changing traffic light, responding to the presence of emergency vehicles, avoiding colliding with a dog who entered into traffic, and maintaining safe driving during a secondary (in-the-car) auditory distraction. Subjects performed the tests sober and then again 30 minutes after smoking a single marijuana cigarette containing either 2.9 percent THC or zero THC (placebo).

Investigators reported that volunteers performed virtually the same after smoking cannabis as they did sober and/or after consuming a placebo. "No differences were found during the baseline driving segment (and the) collision avoidance scenarios," authors reported.

Investigators did note, "Participants receiving active marijuana decreased their speed more so than those receiving placebo cigarette during (the) distracted section of the drive." Authors hypothesized that subjects’ reduction in speed on this task suggested that they may have been compensating for perceived impairment. "[N]o other changes in driving performance were found," researchers concluded.

A 2008 driving simulator study published in the scientific journal Accident, Analysis and Prevention also reported that drivers administered cannabis are likely to decrease their driving speed. "Average speed was the most sensitive driving performance variable affected by both THC and alcohol but with an opposite effect," investigators reported. "Smoking THC cigarettes caused drivers to drive slower in a dose-dependent manner, while alcohol caused drivers to drive significantly faster than in ‘control’ conditions.’"

Previous reviews assessing the crash culpability risk of drivers under the influence of cannabis have reported a positive association between recent marijuana exposure (as typically measured by the presence of active THC in the driver’s blood) and a gradually increased, dose-dependent risk of vehicle accident. However, these studies have consistently found that this elevated risk is below the risk presented by drivers who have consumed legal quantities of alcohol. By contrast, studies have also reported that drivers engaged in the simultaneous use of both cannabis and alcohol can increase their risk of accident compared to the consumption of either substance alone.

NORML’s white paper assessing the impact of marijuana on psychomotor skills, "Cannabis and Driving: A Scientific and Rational Review," is available online at: