The effects of cannabis on driving have been extensively studied for decades. Research consistently shows that typical cannabis use causes very little or no impairment to driving ability.
In terms of public policy, the focus should remain on the real danger, which is alcohol. Cannabis only impairs driving in very high doses, much higher than those usually taken by the vast majority of users. When alcohol users switch to cannabis, our roads get safer.
Here’s a summary from the past 25 years of studies showing that cannabis users are safe drivers.
In 1992, the U.S. Department of Transportation did a study analyzing blood from fatally injured drivers, to see how drugs and alcohol affected collisions. They concluded that “THC-only drivers had a responsibility rate below that of the drug-free drivers. While the difference was not statistically significant, there was no indication that cannabis by itself was a cause of fatal crashes.”
A year later, they confirmed this result with another study on driving simulators, which found that “THC’s adverse effects on driving performance appear relatively small.”
In 1995, the Australia Road Research Unit did a major study into cannabis’ effects on actual driving performance. They found that “THC’s effects on road-tracking after doses up to 300 µg/kg never exceeded alcohol’s at BACs of 0.08% and were in no way unusual compared to many medicinal drugs.”
The researchers also noted that “THC seems to differ qualitatively from many other drugs, in that the users seem better able to compensate for its adverse effects while driving under the influence.”
In 1997, a study into crash characteristics and injuries of victims was published in the Journal of Accident Analysis and Prevention and found that alcohol was more of a problem on the roads than all illegal drugs combined.
“Alcohol is clearly the major drug associated with serious crashes and greater injury. Patients testing positive for illicit drugs (marijuana, opiates, and cocaine), in the absence of alcohol, were in crashes very similar to those of patients with neither alcohol nor drugs. These drugs were not associated with more severe crashes or greater injury.”
In 1998, the British House of Lords Select Committee on Science and Technology issued a report on cannabis and driving. Their studies found that “the impairment in driving skills does not appear to be severe, even immediately after taking cannabis, when subjects are tested in a driving simulator.”
In 1999, University of Toronto researcher Alison Smiley did a “meta-analysis” of studies into cannabis and driving. She concluded that “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.”
Smiley added a pointed commentary. “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.
Another study into the role of cannabis in motor vehicle crashes was published in Epidemiologic Reviews in 1999, and found that cannabis users may even have a reduced risk of accidents. Researchers concluded that “there is no evidence that consumption of cannabis alone increases the risk of culpability for traffic crash fatalities or injuries, and may reduce those risks.”
In 2002, a study into cannabis and alcohol in motor vehicle accidents found that cannabis-users were no more likely to cause accidents than non-users. “In cases in which THC was the only drug present, the culpability ratio was found to be not significantly different from the no-drug group.”
Canada’s Senate released a massive report into all aspects of cannabis in 2002. In regards to driving, they concluded that “cannabis leads to a more cautious style of driving.” The Senators also noted that “cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving.”
The Senate concluded “…cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving.”
In 2007, the Canadian Journal of Public health reviewed several studies into the impact of cannabis on driving. They found that “the severe impact of alcohol on driving abilities are well beyond what has been shown with cannabis.”
A 2010 study into cannabis and driving, printed in the Journal of Psychoactive Drugs, showed that “no differences were found during the baseline driving segment or collision avoidance scenarios,” and concluded that “driving performance was not correlated with highness.”
A study done in 2012 by the Journal of Analytical Toxicology found that even large doses of cannabis produced less impairment than legal levels of alcohol, concluding that there were only “minimal performance changes in critical tracking and divided attention tasks after smoking 700 µg/kg THC. These findings support those documenting minimal impairment in driving-related psychomotor tasks in chronic daily cannabis smokers.”
In 2013, a study into medical marijuana laws, traffic fatalities, and alcohol consumption published in the Journal of Law and Economics showed that legal cannabis means safer roads. They found that “the first full year after coming into effect, legalization is associated with an 8-11% decrease in traffic fatalities.”
The study also found that “legalization is also associated with a sharp decrease in alcohol consumption, which suggests that marijuana and alcohol are substitutes.” The study justified this assertion by including that in, “the first full year after coming into effect, legalization is associated with an 8-11% decrease in traffic fatalities.”
A 2016 study in the American Journal of Public Health confirmed that cannabis access means less traffic accidents, showing that “medical marijuana laws were associated with immediate reductions in traffic fatalities. Dispensaries were also associated with traffic fatality reductions in those aged 25 to 44 years.”
A 2015 study by the U.S. National Highway Traffic Safety Administration, found that cannabis caused much less impairment than alcohol, concluding that “alcohol, but not marijuana, increased the number of times the car actually left the lane and the speed of the weaving.” The NHTSA also analyzed accident patterns, and found that “drivers who tested positive for marijuana were no more likely to crash than those who had not used any drugs or alcohol prior to driving.”
In 2016, the American Automobile Association studied the data on cannabis and driving and concluded that “there was no correlation between blood THC concentration and scores on the individual impairment indicators. Legal limits, also known as per se limits, for marijuana and driving are arbitrary and unsupported by science.”
If the Liberals are going to base their policies on science and research, then the arbitrary 2ng/ml cannabinoid limit for driving needs to be removed. Potential cannabis impairment can be dealt with the same way we do for pharmaceuticals, with warning labels and a standard roadside sobriety test. The evidence shows that legalization of cannabis in Canada is likely to make our roads safer than they are now.