Stoned drivers are safe drivers
The effects of marijuana use on driving performance have been extensively researched over the last 20 years. All major studies show that marijuana consumption has little or no effect on driving ability, and may actually reduce accidents. Here's a summary of the biggest studies into pot use and driving.
A 1983 study by the US National Highway Transportation Safety Administration (NHTSA) concluded that the only significant affect of cannabis use was slower driving - arguably a positive effect of driving high.
A comprehensive 1992 NHTSA study revealed that pot is rarely involved in driving accidents, except when combined with alcohol. The study concluded that "the THC-only drivers had an [accident] responsibility rate below that of the drug free drivers." This study was buried for six years and not released until 1998.
A 1993 NHTSA study dosed Dutch drivers with THC and tested them on real Dutch roads. It concluded that THC caused no impairment except for a slight deficiency in the driver's ability to "maintain a steady lateral position on the road." This means that the THC-dosed drivers had a little trouble staying smack in the center of their lanes, but showed no other problems. The study noted that the effects of even high doses of THC were far less than that of alcohol or many prescription drugs. The study concluded that "THC's adverse effects on driving performance appear relatively small."
A massive 1998 study by the University of Adelaide and Transport South Australia examined blood samples from drivers involved in 2,500 accidents. It found that drivers with only cannabis in their systems were slightly less likely to cause accidents than those without. Drivers with both marijuana and alcohol did have a high accident responsibility rate. The report concluded, "there was no indication that marijuana by itself was a cause of fatal accidents."
In Canada, a 1999 University of Toronto meta-analysis of studies into pot and driving showed that drivers who consumed a moderate amount of pot typically refrained from passing cars and drove at a more consistent speed. The analysis also confirmed that marijuana taken alone does not increase a driver's risk of causing an accident.
A major study done by the UK Transport Research Laboratory in 2000 found that drivers under the influence of cannabis were more cautious and less likely to drive dangerously. The study examined the effects of marijuana use on drivers through four weeks of tests on driving simulators. The study was commissioned specifically to show that marijuana was impairing, and the british government was embarrassed with the study's conclusion that "marijuana users drive more safely under the influence of cannabis."
According to the Cannabis and Driving report, a comprehensive literature review published in 2000 by the UK Department of Transportation, "the majority of evidence suggests that cannabis use may result in a lower risk of [accident] culpability."
The Canadian Senate issued a major report into all aspects of marijuana in 2002. Their chapter on Driving under the influence of cannabis concludes that "Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving."
The most recent study into drugs and driving was published in the July 2004 Journal of Accident Analysis and Prevention. Researchers at the Dutch Institute for Road Safety Research analyzed blood tests from those in traffic accidents, and found that even people with blood alcohol between 0.5% and 0.8% (below the legal limit) had a five-fold increase in the risk of serious accident. Drivers above the legal alcohol limit were 15 times more likely to have a collision. Drugs like Valium and Rohypnol produced results similar to alcohol, while cocaine and opiates showed only a small but "not statistically significant" increase in accident risk. As for the marijuana-only users? They showed absolutely no increased risk of accidents at all.
LINKS AND REFERENCES
1983 National Highway Transportation Safety Administration study: Stein, AC et al., A Simulator Study of the Combined Effects of Alcohol and Marijuana on Driving Behavior-Phase II, Washington DC: Department of Transportation (1983)
1992 National Highway Transportation Safety Administration study: The Incidence and Role of Drugs in Fatally Injured Drivers, by K.W. Terhune, et al. of the Calspan Corp. Accident Research Group in Buffalo, NY (Report # DOT-HS-808-065)
1993 National Highway Transportation Safety Administration study: Marijuana and actual Driving Performance, By Hindrik WJ Robbe and James F O'Hanlon. Institute for Human Psychopharmacology, University of Limburg
1998 University of Adelaide and Transport South Australia study:
1999 University of Toronto Study, Marijuana Not a Factor in Driving Accidents:
2000 UK Transport Research Laboratory study on Cannabis and Driving:
2000 UK Department of Transportation's Cannabis and Driving report:
2002 Report of the Special Senate Committee on Illegal Drugs
July 2004, Journal of Accident Analysis and Prevention, Psychoactive substance use and the risk of motor vehicle accidents.
For a less scientific and more amusing study of the combination of drugs and driving, go here:
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MEDIA REPORTS ON "DRUGGED DRIVING" LAWS
Drug Office Out To Convince Teens Pot Impairs Driving
Lexington Herald-Leader, December 3, 2004
Growing danger: Drugged driving
USA Today, Oct 21, 2004
Lawmakers Aiming for 'Zero Tolerance' Of Pot-Smoking Drivers
The Athens News, May 5, 2004
New Legislation To Allow Police To Conduct Roadside Tests for Drug Impaired Drivers
Ottawa Citizen, February 23, 2004
Drug Czar, Prohibition Establishment Seek 'Zero Tolerance' for 'Drugged Driving'
The week online with DRCNet, November 22, 2002
"Steer Clear of Pot" Media Campaign
US Office of National Drug Control Policy