EXECUTIVE SUMMARY

Collisions remain a major cause of death and injury in Canada. Concerns about the role of cannabis in collision causation date back many years, although much less is known about the impact of this drug on collisions than alcohol. Among the reasons for this has been the much greater difficulty involved in measuring the presence and amount of cannabinoids compared to alcohol. However, 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. The purpose of this document is to provide an overview of available research and evidence on the potential impact of cannabis on road safety in Canada. Six areas of relevance to this issue are 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.

A substantial body of research assessing the effects of cannabis on human performance exists, and several authoritative reviews of this literature have been published. The evidence is very clear 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.

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.

Information on cannabis use and driving is unfortunately rare, but some data are available. In the general driving population, the proportion that report driving after cannabis use in the previous year is low, with recent estimates ranging from 1.5% to 1.9%. However, it is clear that there are subgroups of the driving population for whom driving after cannabis use is much more common. Cannabis users and younger drivers are much more likely to report driving after cannabis use, and a recent study of Ontario students found that 19.3% of drivers in high school reported driving within an hour of using cannabis in the previous year.

Many studies internationally have examined the prevalence of cannabinoids in drivers who have been injured or killed in motor vehicle collisions. These studies reveal that cannabinoids are the drugs most commonly found after alcohol in these individuals. Evidence from Canada is consistent with this observation; two recent studies detected cannabinoids in 13.9% and 19.5% of samples of seriously injured and fatally injured drivers, respectively. For several methodological reasons it is much more difficult to assess the contribution of cannabis to collision risk, for example, it is very difficult to obtain appropriate control samples not involved in collisions. In the most methodologically sound study reported to date, Quebec researchers found that cannabis was associated with a doubling of the risk of being involved in a fatal collision. However, other studies present more variable results. Other research paradigms can also provide important evidence here. For example, recent studies with individuals seeking treatment for substance abuse have found that individuals who report a problem with cannabis have elevated collision histories in the few years preceding treatment entry.

When cannabis enters the body, THC and other cannabinoids are widely distributed to all tissues. Cannabinoids accumulate in fatty tissues and are slowly released into other body compartments. Metabolites can be found in the urine for up to 12 days after a single dose of THC and for a month or two after heavy use. For this and other reasons research has frequently observed a poor correlation between plasma or urine concentrations and the pharmacological effects of the drug. However, more recent studies have been able to link plasma concentrations with pharmacological effects with more accuracy. Several methods have been proposed or used to assess the presence of cannabinoids in drivers, including breath, blood, urine, saliva and sweat tests, and behavioural examinations. While blood tests are the ‘gold standard,’ they are very invasive and create logistic and legal concerns. Urine tests may not differentiate between cannabis use that occurred very recently and use that occurred days or even weeks before. Saliva and sweat tests and behavioural examinations are promising procedures that are being used or assessed in many jurisdictions.

The Criminal Code of Canada permits police to lay a charge of impaired driving (section 253a) if they believe a person’s ability to operate a vehicle is impaired by ‘alcohol or a drug’. British Columbia and some other provinces have begun, or plan to begin, training police officers as Drug Recognition Experts. Canada’s current legal approach to cannabis and driving is similar to that taken in many jurisdictions. However, in several jurisdictions legislation has been planned or introduced to create specific offences of driving under the influence of cannabis and other drugs (e.g., Germany, Belgium, New York, Nevada, New South Wales). 

How to get the full report:

The full report on the Impacts of cannabis on driving: An analysis of current evidence with an emphasis on Canadian data is available in the following formats:

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