NCPA - National Center for Policy Analysis

Smoking Marijuana Makes You A Worse Driver

February 13, 2001

In a British study, the Transport Research Laboratory (TRL) Safety Department investigated the effect of marijuana intoxication on driving ability. In the study, 15 volunteers completed driving tests in a simulator while under the influence of a range of doses of marijuana. Researchers measured their accuracy at steering the car, known as "tracking ability," and other psychomotor responses, such as hazard perception and braking responses. They took blood and saliva samples at regular intervals and also tested the subjects' coordination, balance and timing.

The principal findings were that:

  • The subjects drove more slowly under the influence of marijuana, compensating for their intoxication by driving more cautiously.
  • Tracking ability was the only test criterion that was adversely affected -- that is, the volunteers found it very difficult to follow a figure-of-eight loop of road when given a high dose.
  • Reaction times to motorway hazards and performance on cognitive tests in the lab were not significantly affected.

Marijuana is by far the most common illegal drug found in the bloodstream of road accident victims -- mainly because THC, the active ingredient in marijuana, can remain in the body for more than a month while alcohol is quickly removed within hours.

Source: Arran Frood, "Don't Be A Dope. If You're Going To Smoke A Joint, Don't Drive Home," New Scientist, 16 December 2000.


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