Pot’s Effects On Driving Performance Contrast Alcohol’s, Study Says

Jerusalem, Israel: Low doses of cannabis and alcohol have contrasting effects upon psychomotor performance, according to clinical trial data published in the current issue of the journal Accident Analysis and Prevention.

Investigators at Hebrew University and the University of the Negev in Israel assessed the impact of alcohol and THC on simulated driving performance in fourteen subjects.

Researchers reported that volunteers’ subjective and actual performance differed under the influence of THC compared to alcohol.

“Average speed was the most sensitive driving performance variable affected by both THC and alcohol but with an opposite effect,” authors wrote. “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.”

Both alcohol and low doses of cannabis impaired drivers’ ability to maintain lane position and significantly increased subjects’ reaction time. Neither low doses of alcohol nor THC significantly increased subjects’ total number of collisions.

In terms of overall driving performance, subjects administered cannabis performed in a manner similar to drivers with a blood alcohol concentration of 0.05, authors determined.

“The present study reveals that although some similarities in the degree of impairment could be observed – mainly with the lower level of THC and alcohol, where both increased reaction time and [lane position variability] – some discrepancies also appeared between the two drugs,” authors concluded. “In particular, subjects seemed to be aware of their impairment after THC intake and tried to compensate by driving slower; alcohol seemed to make them overly confident and caused them to drive faster than in control sessions.”

Two recent examinations of fatal accident crash data indicate that alcohol, even at low doses, greatly increases drivers’ crash risk compared to cannabis. A 2007 case-control study published in the Canadian Journal of Public Health reported that US drivers with blood alcohol levels of 0.05 percent were three times as likely to have engaged in unsafe driving activities prior to a fatal crash as compared to individuals who tested positive for marijuana. Similarly, a 2005 review of French auto accident data reported that drivers who tested positive for any amount of alcohol had a four times greater risk of having a fatal accident than did drivers who tested positive for marijuana in their blood.

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. Full text of the study, “Effects of THC on driving performance, physiological state, and subjective feelings relative to alcohol,” appears in Accident Analysis and Prevention. Additional information regarding marijuana use and on-road accident risk is available in the NORML report “Cannabis and Driving: A Scientific and Rational Review,” available online at: http://www.norml.org/index.cfm?Group_ID=7459.