May 12, 2004

Government releases report on impacts of marijuana on driving

Ottawa, Ontario – The government of Canada has released an analysis of current evidence on the impact of cannabis on driving safety. The report was prepared for Road Safety and Motor Vehicle Regulation, Transport Canada by Robert E. Mann, PhD, Bruna Brands, PhD, Scott Macdonald, PhD, and Gina Stoduto, MEd.

“There is renewed interest in this issue, stimulated in part by proposed legislative changes on the part of the Government of Canada to decriminalize possession of small amounts of cannabis,” said the report.

Six areas of relevance were considered:

  1. research on the effects of cannabis on the skills necessary for safe driving

  2. research on the prevalence of cannabis use in Canada
  3. research on the prevalence of driving after cannabis use in Canada
  4. epidemiological studies of the impact of cannabis on collision risk
  5. means for assessing the presence of cannabis in drivers, and
  6. legal initiatives in other jurisdictions to address the issue of cannabis and driving.

According to the report, after alcohol, cannabis is the most widely used psychoactive drug in Canada. Use of the cannabis was relatively uncommon until the 1960s, and since then has increased substantially. In Canada, only a small number of national surveys have examined cannabis use. In a 1994 survey of Canadians aged 15 and over, about 1/3 of respondents reported using cannabis at some point in their lives while 7.3% reported using cannabis in the previous year. Current usage rates were highest in British Columbia and lowest in Ontario. Trend data from Ontario reveal that cannabis use has been increasing among high school students since 1991, and has reached levels last seen in the late 1970s. Among adults the trends are much less clear, although the proportion of users in recent years is higher than observed in the early 1990s.

The report states that a moderate or higher dose of cannabis impairs driver performance and several of the skills necessary for safe driving. Some authors have reported that the largest degree of impairment is observed with tasks involving attention, tracking and psychomotor skills. As with alcohol and other psychoactive drugs, tolerance may be observed to some of the effects of cannabis in experienced users. The effects of using cannabis in combination with alcohol, which seems to occur frequently among cannabis users, appear to be either additive, in which the effects are roughly equivalent to adding the effects of the two drugs together, or multiplicative, in which the effects of the drugs taken together are greater than an addition of the effects of the two drugs.
A full copy of the report is available online at

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