The safety hazards of smoking marijuana and driving are overrated, says University of Toronto 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 Healt
Contact: Alison Smiley
University of Toronto