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Driving: Cannabis has little effect on driving

Driving under the influence of any drug is generally discouraged, but studies have always indicated that marijuana (cannabis) has only a neglible effect on drivers who are experienced with its effects. The reason seems to be that, while there is a minor reduction in reaction times similar to being a few years older than the driver's current age, there is a sense of "paranoia" that leads to slower and more cautious driving. The most common effect is getting lost.

Remember: Never Drive when impaired.

Marijuana Has Less Adverse Effect on Driving Than Alcohol, Tiredness, U.K. Study Says

Crowthorne, Berkshire, United Kingdom: Marijuana appears to have less adverse impact on driving ability than does alcohol, according to findings from a recent study by the U.K.'s Transport Research Laboratory (TRL). The results replicate earlier findings recorded in the U.S., Australia and elsewhere indicating that marijuana intoxication plays a relatively insignificant role in vehicular accidents.

NORML Foundation Director Allen St. Pierre said the results were not surprising. "Study after study shows that marijuana's slight impairment on psychomotor skills generally falls within the range of safety we accept for prescription medications and other legal, potentially debilitating factors; the findings of this latest inquiry are no different."

The TRL study examined the driving performance of fifteen volunteers while under the influence of low and high doses of marijuana, and while sober. All volunteers were tested using a sophisticated driving simulator. Researchers found that marijuana appeared to adversely influence subjects' ability to accurately steer a car (so-called "tracking ability"), but found their reaction time and all other measures of driving performance to be unaffected by the drug. Researchers further noted that subjects were cognizant of their impairment and "attempt[ed] to compensate for [it] by reducing the difficulty of the driving task, for example by driving more slowly."

The authors concluded: "In terms of road safety, it cannot be concluded that driving under the influence of cannabis is not a hazard. ...However, in comparison with alcohol, the severe effects of alcohol on the higher cognitive processes of driving are likely to make this more of a hazard, particularly at higher levels."

Similar trials previously conducted by the TRL have shown that alcohol and sleep deprivation have a more adverse impact on driving ability than does marijuana. Tests from other countries have yielded comparable results. A May 1998 Australian review of 2,500 injured drivers reported that cannabis had "no significant effect" on driving culpability. A pair of studies released by the U.S. National Highway Traffic Safety Administration in 1992 and 1993 found the adverse effects of marijuana on driving "relatively small," and concluded that "there [was] no compelling evidence that marijuana contributes substantially to traffic accidents or fatalities."

The most recent TRL study was commissioned by the British Department of Environment, Transport and the Regions.

For more information, please contact Allen St. Pierre, NORML Foundation Executive Director at (202) 483-8751. Copies of the study, entitled: "The influence of cannabis on driving," are available online at:


Cannabis and Driving

A study performed for the UK Department of the Environment, Transportation and the Regions by the Transport Research Laboratory, "The Influence of Cannabis on Driving," is available for download in PDF format from this URL:

Or a web version is available at

The study concludes that though cannabis does have an effect on driving ability, that alcohol is much worse, and that marijuana users notice their impairment and compensate by driving slower and more carefully.

The influence of cannabis on driving

by B F Sexton, R J Tunbridge, N Brook-Carter (TRL Limited), P G Jackson (DETR), K Wright (University of Birmingham), M M Stark (St George's Hospital Medical School) and K Englehart (Principal Police Surgeon) The results from a study of different doses of cannabis and the influence on driving and driving related skills are reported. The study required participants who were male, drivers and regular cannabis users to undertake a variety of different tasks.

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