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Marijuana and Driving: A Review of the Scientific Evidence
It is well established that alcohol increases accident risk. Evidence of marijuana’s culpability in on-road driving accidents is much less convincing.
Although cannabis intoxication has been shown to mildly impair psychomotor skills, this impairment does not appear to be severe or long lasting. In driving simulator tests, this impairment is typically manifested by subjects decreasing their driving speed and requiring greater time to respond to emergency situations.
Nevertheless, this impairment does not appear to play a significant role in on-road traffic accidents. A 2002 review of seven separate studies involving 7,934 drivers reported, “Crash culpability studies have failed to demonstrate that drivers with cannabinoids in the blood are significantly more likely than drug-free drivers to be culpable in road crashes.” This result is likely because subject under the influence of marijuana are aware of their impairment and compensate for it accordingly, such as by slowing down and by focusing their attention when they know a response will be required. This reaction is just the opposite of that exhibited by drivers under the influence of alcohol, who tend to drive in a more risky manner proportional to their intoxication.
Today, a large body of research exists exploring the impact of marijuana on psychomotor skills and actual driving performance. This research consists of driving simulator studies, on-road performance studies, crash culpability studies, and summary reviews of the existing evidence. To date, the result of this research is fairly consistent: Marijuana has a measurable yet relatively mild effect on psychomotor skills, yet it does not appear to play a significant role in vehicle crashes, particularly when compared to alcohol. Below is a summary of some of the existing data.
(For more information on NORML’s position regarding marijuana, driving and the law, please click here or visit NORML’s Principles of Responsible Cannabis Use.)
Marijuana Not a Factor in Driving Accidents
March 29, 1999
The safety hazards of smoking marijuana and driving are overrated, says U of T researcher Alison Smiley.
Recent research into impairment and traffic accident reports from several countries shows that marijuana taken alone in moderate amounts does not significantly increase a driver's risk of causing an accident -- unlike alcohol, says Smiley, an adjunct professor in the department of mechanical and industrial engineering . While smoking marijuana does impair driving ability, it does not share alcohol's effect on judgment. Drivers on marijuana remain aware of their impairment, prompting them to slow down and drive more cautiously to compensate, she says.
"Both substances impair performance," Smiley says. "However, the more cautious behaviour of subjects who received marijuana decreases the drug's impact on performance. Their behaviour is more appropriate to their impairment, whereas subjects who received alcohol tend to drive in a more risky manner."
Smiley, who has studied transportation safety for over 25 years, drew her results from a "metanalysis" of existing research into the effects of marijuana on driving ability, combined with traffic accident statistics in the United States and Australia. Previous studies showing stronger effects often combined "fairly hefty doses" by researchers with driving immediately after consumption, likely exaggerating the drug's effects, she believes.
While Smiley does not advocate legalizing the drug, she says her results should be considered by those debating mandatory drug tests for users of transportation equipment such as truck or train drivers, or the decriminalization of marijuana for medical use. "There's an assumption that because marijuana is illegal, it must increase the risk of an accident. We should try to just stick to the facts."
Smiley presented her findings at a symposium of the American Academy of Forensic Sciences in Florida in February. Her paper was also published in Health Effects of Cannabis, a publication of Toronto's Centre for Addiction and Mental Health, in March.
Professor Alison Smiley
department of mechanical and industrial engineering
Stoned drivers are safe drivers
by Dana Larsen (11 Jan, 2005) Two decades of research show that marijuana use may actually reduce driver accidents.
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:
A BETTER WAY TO TEST
Performance testing is better than drug testing
Cannabis Culture, January 2005
Alternatives to Drug Testing: Performance testing Non-testers List
Performance testing can add an extra measure of safety
HR Magazine, February 1996
An Alternative to Drug Testing
Inc Magazine, April 1995
MEDIA REPORTS ON "DRUGGED DRIVING" LAWS
UK Launches Drug Driving Tests
Daily Telegraph, December 22, 2004
Drug Office Out To Convince Teens Pot Impairs Driving
Lexington Herald-Leader, December 3, 2004
Growing danger: Drugged driving
USA Today, Oct 21, 2004
Zero-tolerance drugged driving law doing the job
The Daily Press, July 8, 2004
Lawmakers Aiming for 'Zero Tolerance' Of Pot-Smoking Drivers
The Athens News, May 5, 2004
Drugged Driving Statutes Pushed
Boston Globe, March 21, 2004
New Legislation To Allow Police To Conduct Roadside Tests for Drug Impaired Drivers
Ottawa Citizen, February 23, 2004
Too Many One Toke Over Line, Police Say
Globe and Mail, February 1, 2003
Drug Czar, Prohibition Establishment Seek 'Zero Tolerance' for 'Drugged Driving'
The week online with DRCNet, November 22, 2002
British Police Plan New Drug Tests For Drivers
Reuters, August 3, 2000
Marjiuana Report Too Hot Too Handle
Australian Broadcasting Corporation, October 1998
"Steer Clear of Pot" Media Campaign
US Office of National Drug Control Policy
Drivers on Pot - Issues and Options
Who's using pot? / How does pot affect drivers? / Cannabis and Crashes / Legal Options
The UK government is reclassifying cannabis this year. While the drug remains illegal, it is now classed with amphetamines and barbiturates, rather than heroin. The advisory council which recommended the change stressed that cannabis is unquestionably harmful and was anxious that the public be informed of the dangers associated with its use. A public education campaign is being implemented as part of that country's drug strategy.
Canada's federal government is moving in a somewhat similar direction. In response to recommendations from a Senate special committee on illegal drugs, it wants to decriminalize possession of small amounts of cannabis. Although the substance would still be illegal, this proposal has raised concerns about increased use, which may lead to more drivers impaired by marijuana.
Under the Criminal Code of Canada, drivers can be charged if impaired by alcohol or drugs. However, there is no per se offense for substances other than alcohol. Criminal charges can be laid if a driver's blood alcohol concentration (BAC) is 0.08 or higher. Above that level, the likelihood of a crash is known to increase substantially. Most Canadian jurisdictions impose administrative licence suspensions at BACs of 0.05 or lower.
Breathalyzers provide an easy, effective and convenient way for police to detect and measure the presence of alcohol. However, at present there is no reliable and non-intrusive rapid roadside method to test for pot. Even if a "potalyzer" were available, a defensible per se limit must be set, at which a pot-using driver is criminally impaired.
In Canada, some police services have officers trained specifically as drug recognition experts, who can determine if a driver is impaired by drugs. The Senate special committee felt this visual recognition method has yielded satisfactory results.
Who's using pot?
The Traffic Injury Research Foundation (TIRF) found that 1.5 per cent of drivers surveyed in its 2002 Road Safety Monitor had used marijuana within two hours of taking the wheel. The survey found that young men are most likely to drive after using marijuana or other illegal drugs. One-third of those who drove after using marijuana also drove after drinking.
An Ontario study released in March 2003 showed 15 per cent of students in grades 10 to 13 who had a driver's licence reported driving within an hour after consuming two or more drinks. Even more, 20 per cent, reported driving within an hour after using cannabis.
The Canadian Centre on Substance Abuse (CCSA) says cannabis use by Canadian teens is among the highest in the world. In Canada (as well as Australia, France, Ireland, UK and the US) more than 25 per cent of all high school students report past-year use.
In its September 2002 report, the Senate special committee concluded that between five and 12 per cent of drivers may driver under the influence of cannabis; this percentage increases to over 20 per cent for young men under 25 years of age.
How does pot affect drivers?
The psychoactive chemical in marijuana is tetrahydrocannabinol (THC). THC has a very different effect from alcohol. Pot users are acutely aware of their impairment - that is, they feel "high" - and some try to compensate by driving more cautiously.
THC diminishes psychomotor skills and attention span. It reduces the ability to perform tracking tasks; at high doses, users drive less accurately and show difficulty with steering. Alcohol additionally impairs cognitive function, including risk perception, decision-making and planning. It can also trigger aggressive driving behaviour such as speeding and following too closely.
For alcohol, levels of impairment have been correlated with specific concentrations in the blood. No such correlation is available for THC. Complicating the issue, THC can be detected in the body for up to four weeks, although its impairing effects do not last.
Studies indicate that the effect of cannabis use on driving is related to the dose. Some claim that a single glass of wine will impair driving more than smoking a joint. However, the potency of a joint can vary considerably. Driver use of cannabis is of particular concern when combined with alcohol. The combination increases a drinking driver's risk of being responsible for a crash.
Further research is needed in order to establish a THC level at which the substance criminally impairs driving ability, and how to assess drivers.
In January 2002, the European Commission initiated a three-year project with the acronym IMMORTAL (Impaired Motorists, Methods of Roadside Testing and Assessment for Licensing). The results of this study will provide a much-needed scientific base for legislation.
Cannabis and Crashes
A significant number of drivers killed in road crashes have a combination of drugs and alcohol. "Drugs" include cannabis and other illicit substances as well as prescription and over-the-counter medications.)
A TIRF study of Ontario driver fatalities in the early 1980s determined that over half of all the deceased who tested positive for drugs had also been drinking.
A similar 1990 study found alcohol in 57 per cent and cannabis in 11 per cent of driver fatalities. Of the cases that were positive for cannabis in the blood, 84 per cent also tested positive for alcohol and 70 per cent were males under age 25.
In a British Columbia study of drivers killed in 1990 and 1991, 48 per cent tested positive for alcohol. Drugs were found in 20 per cent of all cases; cannabis alone accounted for 13 per cent. Over half of drivers who tested positive for any drugs had also been drinking.
A 2002 Quebec study showed drugs are implicated in a substantial number of fatal crashes. Cannabis was detected in 19.5 per cent of driver fatalities. Alcohol was found in 41 per cent of all drug-positive cases. In the roadside sample, drugs were found in 11.8 per cent of drivers, including 6.7 per cent who had cannabis. Alcohol was detected in 5.1 per cent. Alcohol and drugs were found together in 5.9 per cent of drug-positive cases.
A UK Department for Transport study published in 2000 revealed that traces of illicit drugs were present in 18 per cent of road fatalities in that country, a six-fold increase from the mid-1980s. Cannabis was found in two-thirds of these cases.
France has passed a law making it a crime to drive under the influence of drugs. The law foresees a two-year prison sentence and a € 4,500 fine for anyone driving or accompanying someone driving "under the influence of plants or substances classified as narcotics." These measures are above and beyond those for drunk driving. Critics say it will be very hard to implement the new law. Police officers may have a hard time getting suspected drug users to a medical clinic or hospital for blood or urine tests.
This zero tolerance approach would not likely work in Canada. French law, unlike Canadian law, assumes the accused is guilty and is not as open to court challenges.
Other EU countries take varying approaches to drug-impaired driving. In Germany and Spain, detection of any trace of a drug gives rise to an administrative offence, but there must be proof of impaired driving skills for a criminal charge to be applied.
In January 2003, a judge in Pembroke, Ontario acquitted a man charged with driving while impaired by marijuana. The accused man had a medical exemption to smoke marijuana as a treatment for his multiple sclerosis. The judge could not tell what caused the accused to swerve over the centre line when driving, and to slur his speech and lose his balance when police pulled him over. It could have been the pot smoke, the illness or some other factor.
Conviction under the Criminal Code of Canada requires proof 'beyond a reasonable doubt,' so Canadian legislators must weigh their alternatives carefully. In the absence of definitive research on how cannabis use relates to road crashes, court challenges may hinder conviction in criminal cases. Until sufficient evidence is available, sanctions outside the Criminal Code should be seriously considered.
Administrative licence suspensions have proven to be an effective tool in the fight against impaired driving. Most provinces impose 12-to 24- hour suspensions under their traffic codes on drivers whose BAC is below the 0.08 criminal limit. These suspensions remove potentially dangerous drivers from the road. They provide a stern and effective warning without the punitive lifetime consequences of a criminal record and a costly criminal court case.
The Canada Safety Council urges provincial and territorial governments to consider imposing administrative licence suspensions on drivers who have been using cannabis. Police with reason to believe a driver has been smoking pot should be able to suspend that driver's licence without a criminal charge. If alcohol is also involved, appropriate action would be taken depending on the BAC.
The prevalence of cannabis use among younger, mostly male, drivers raises cause for concern. Precautionary action is needed - but the research to date does not support zero tolerance with automatic criminal sanctions.
July 24, 2003
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