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Major Studies of Drugs and Drug Policy
Canadian Senate Special Committee on Illegal Drugs
Volume I - General Orientation

Chapter 8

Driving under the influence of cannabis[1][1]

Stan Thompson was 18 when he and four other teenagers from Kanata were killed in a horrific car accident near Perth that summer day. A youth was found responsible for the fatal accident and served eight months of a 12-month sentence. Cannabis and alcohol-impaired driving was the cause. … The year following Stan's death, his father, Greg Thompson, went to local high schools to talk about the tragedy. He spoke to students about what went wrong and how the tragedy could have been prevented. … His message was that driving a vehicle and smoking marijuana does and will affect driving abilities. He pleaded with the kids not to do it. … Cannabis is not a benign substance. There is very little in the way of research that allows anyone to determine levels of impairment related to cannabis and driving ability, much less the levels of impairment related to cannabis and alcohol and driving ability. We have seen in the Manitoba survey, over one-half of the kids that are using cannabis do so in cars and during school hours. There is no technical or scientific ability to test for cannabis impairment. We do not have the technology, scientific data or the research. We do not have the proper legislation. Studies done in British Columbia indicate that 12 per cent to 14 per cent of the drivers involved fatal motor vehicle accidents had cannabis in their systems. The Government of Quebec and the insurance board in Quebec are presently doing road surveys where people are voluntarily submitting to urine or blood tests. The findings in these tests are that between 12 per cent and 14 per cent of those drivers has cannabis in their system while driving. [2][2]

If there is one issue, other than the effects of cannabis use on young people or the effects of substance abuse, that is likely to be of concern to society and governments, then it is certainly the issue of how it affects the ability to drive a vehicle. We are already familiar with the effects of alcohol on driving, and the many accidents involving injuries or deaths to young people. In spite of the decreases in use noted in recent years, it is not difficult to admit that one fatal accident caused by the use of a substance is already one accident too many.

As it happens, after alcohol, cannabis is the most widely used psychoactive substance, particularly among young people in the 16-25 age group. Casual use occurs most often in a festive setting, at weekend parties, often also accompanied by alcohol. People in this age group are also the most likely to have a car accident and are also susceptible to having an accident while impaired.

We have seen that cannabis affects psychomotor skills for up to five hours after use. The psychoactive effects of cannabis are also dependent on the amount used, the concentration of THC and the morphology, experience and expectations of users. But what are the specific effects of cannabis on the ability to drive motor vehicles? What are the effects of alcohol and cannabis combined? And what tools are available to detect the presence of a concentration of THC that is likely to significantly affect the psychomotor skills involved in vehicle operation?

Here again, the witnesses heard by the Committee vary in their interpretation of the study results. Thus, the Canadian Police Association told us:


Driving while intoxicated by drugs impairs judgment and motor coordination. In one study involving aircraft 10 licensed pilots were given one marijuana joint containing 19 milligrams of THC - a relatively small amount. Twenty-four hours after smoking the joint, they were tested in a flight simulator. All 10 of the pilots made errors in landing and one missed the runway completely. [3][3]


Two weeks later, Dr. John Morgan of the City University of New York Medical School said in reference to the same study:


A California-based scientist named Jerome Yesavage wrote the study. It was done in the early 1980s, I think, and it attracted enormous attention. … Doctor Yesavage's study … was completely uncontrolled. … As you all have heard, it is difficult to control for marijuana use. When Doctor Yesavage was funded by the federal government to repeat the study with the simple controls that others and I had suggested, they were unable to show any impact of marijuana use after four hours in a similar group of people. Therefore, I believe that the truth is that marijuana use will impact airplane and driving simulators and to some degree driving performance for three hours to four hours after use; however there is no sustained impact. Any impact is relatively minor. [4][4]


Making reference to Robbe’s work, which we will be examining in greater detail in this chapter, Professor Morgan added:


A Dutch scientist who has for years worked on driving experiments found that marijuana using drivers have a little difficulty staying right in the middle of the road. That is most sensitive test. If you smoke marijuana, you tend to weave a little bit more than completely sober people do. That is important, although there have been no studies to show that that amount of weaving had a gross impact on driving ability. 

The Dutch scientist included in his report that the amount of weaving was approximately the same in individuals consuming very small amounts of alcohol, very small doses of bensodiazopenes and very small doses of antihistamines. [5][5]


On the same day, Professor Kalant of the University of Toronto responded as follows:


Dr. Morgan referred to some experimental studies this morning. A number of studies, reviewed by Dr. Smiley in the report of the World Health Organization Committee on Health Effects of Cannabis, indicate a fair measure of agreement on what the predominant effects on driving are. The lane control, as Dr. Morgan mentioned, is impaired. The person does not steer as accurately. In addition, there was slower starting time and slower braking time. There was decreased visual search. In other words, when you drive, you must monitor for sources of danger to both sides and not just ahead of you. There was decreased monitoring, decreased recognition of danger signals. The effects were synergistic with those of alcohol. The one favourable thing about cannabis compared with alcohol was that there was less aggressiveness in the cannabis smokers than in the drinkers, so they were less likely to pass dangerously or to speed. Nevertheless, driving ability was impaired not just by weaker, poorer steering control, but also by less alertness to unexpected things that might happen and pose a hazard. 

I will not go into the statistics of actual field studies of the involvement of cannabis in driving accidents. However, I would like to say that a number of studies have shown that there has been evidence of cannabis presence in the blood or the urine of people who have been stopped for impaired driving who did not have alcohol present. [6][6]


As we can see, and as was the case with respect to the effects and consequences on the health of users, there are divergent opinions about the interpretation of studies and their meaning in connection with the specific effects of marijuana on driving.

This chapter is divided into three sections. The first considers the ways of testing for the presence of cannabinoids in the body. The second analyses studies on the known prevalence of impaired driving, in both accident and non-accident contexts. The third and last summarizes what is known about the effects of cannabis on driving based on both laboratory and field studies. As in the other chapters, the Committee will then draw its own conclusions.




[1][1]  In addition to the specific studies we consulted, which will be referred to appropriately, this chapter is largely based on the surveys carried out by INSERM (2001) op. cit., Ramaekers et al., for the International Science Conference on Cannabis in Pelc, I., op. cit.), and Smiley (1999) in Kalant (ed.) op.cit.

[2][2]  R.G. Lesser, Chief Superintendent, Royal Canadian Mounted Police, testifying before the Special Senate Committee on Illegal Drugs, October 29, 2001, Issue 8, page 17.

[3][3]  Dale Orban, Detective Sergeant, Regina Police Service, for the Canadian Police Association, testimony given before the Special Senate Committee on Illegal Drugs, May 28, 2001, Issue 3, page 47.

[4][4]  Dr John Morgan, Professor at the City University of New York Medical School, testimony before the Special Senate Committee on Illegal Drugs, June 11, 2001, Issue 4, page 40-41.

[5][5]  Ibid.

[6][6]  Dr Harold Kalant, Professor Emeritus, University of Toronto, testimony before the Special Senate Committee on Illegal Drugs, June 11, 2001, Issue 4, page 75.

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