NOTE: The NORML Canada medical page contains a large amount of additional information, such as numerous reports on scientific studies which indicate the degree of danger produced by marijuana on drivers, if any.
FACT: SCIENTIFIC TESTS OVER 2 AND 1/2 DECADES HAVE NOT SUBSTANTIALLY CONFIRMED THIS THEORY. In fact, marijuana use may slightly improve driving performance, according to science. Two studies (one indexed below) have shown a negative correlation between marijuana and accidents.

Marijuana and Driving

by the web maintainer.
... "The more dope I smoked, the better I drove"... Don Sherman, Car and Driver writer, June 1980.
Sherman's statement was based upon scientific measurement, not superstition, religious fanaticism, or other biased opinion.

My introduction to marijuana and driving occurred when I read an article about that subject in the June 1980 issue of Car and Driver magazine. The authors of this article deliberately chose from the very beginning to not even consider the possibility that anyone could be driving responsibly while under the influence of marijuana.

But when yielding their own scientific test results, and when quoting the U. S. government's own test results, concluded that motivated drivers, stoned on marijuana, were about as good as unstoned drivers in the same tests, although some stoned drivers actually improved a bit when they weren't goofing off, sleeping, or looking at the clouds. (To arrive at most of the rational conclusions of the article, one must go to the very end section.)

The scientific tests mentioned in the article were quoted by Dr. Alan C. Donelson of the Highway Safety Research Institute in Ann Arbor, Michigan. I didn't see the older studies on the internet, but later studies are readily available :

Some of the Conclusions of an Official U. S. Government Study on Driving and Marijuana :

(These are not the studies mentioned in the June 1980 Car and Driver magazine.)

DOT HS 808 078 NOVEMBER 1993

For the entire study : click here .

"... Of the many psychotropic drugs, licit and illicit, that are available and used by people who subsequently drive, marijuana may well be among the least harmful. Campaigns to discourage the use of marijuana by drivers are certainly warranted. But concentrating a campaign on marijuana alone may not be in proportion to the safety problem it causes. ..."

  • The maximum road tracking impairment after the highest THC dose (300 ug/kg) was within a range of effects produced by many commonly used medicinal drugs and less than that associated with a blood alcohol concentration (BAC) of 0.08g% in previous studies employing the same test.
  • It is not possible to conclude anything about a driver's impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample.

Some studies and misc. resources :

NORML Canada Comments

NORML Canada Editorial Position on Marijuana and Driving : NORML Canada does not believe that testing drivers for violation of the law, using the presence of THC in the blood as the basis, has validity since there is no correlation between marijuana induced impairment and THC measured in the blood, according to research.

Until driving violation citations are based upon measurable impairment (ostensibly produced by marijuana, or synthetic versions of it), NORML Canada opposes the current laws and enforcement against driving under the influence of marijuana, using blood tests for THC as the testing method, or any other simple measure of THC in the body, including breath tests, unless measurements for THC from such tests have been proven to correlate with driving impairment. UMBERTO IORFIDA, PREZ.  

More cautious drivers ? Although marijuana definitely impairs driving in laboratory test conditions, the amount of impairment produced in real-life city traffic conditions was minimal or non-existent! The amount of impairment measured in the lab never exceeded the impairment produced by .08% Blood Alcohol Level (or BAL), the minimum measurment of alcohol produced impairment necessary to produce a DWI or DUI arrest in most areas. The main danger could be the possibility of mixing alcohol and marijuana. It is possible that marijuana makes the effect of alcohol a little worse. How worse, the study did not quantify.
(It is common knowledge among the coffeeshop managers in Amsterdam that alcohol and marijuana are not a good combination for many people, especially the inexperienced in either drug. Many coffeeshops turned to selling beer on tap in Amsterdam in order to make enough money to stay in business, greatly shocking many of the early "harm reduction" proponents there. Many considered this a "sell out" to the "hard drug pushers", the alcohol industry.)
The U. S. researchers, even in the first paragraph of the study, were quick to point out that the effects of marijuana, even in high doses, never caused the subjects to pose a safety hazard to themselves or to other traffic. The U.S. and Dutch researchers both used actual driving conditions for parts of the study, with no fear of the consequences. Why? Is the threat of marijuana mostly an imaginary threat?

The possibility that governments world wide will soon begin to arrest and fine drivers specifically for the offense of having smoked marijuana in the last 30 days, even though the official government research shows that marijuana produces short lasting minimal impairment, well within the range of accepted legal pharmaceuticals, is another poorly reasoned response to a possibly non-existent problem.

People who are planning to be irresponsible in one area of life, may be irresponsible in other areas. If marijuana does not significantly impair driving ability, it should not be associated with irresponsibility.

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