Your guide to making money in the multi-billion dollar marijuana industry
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| Major Studies of Drugs and Drug Policy | ||||
| Canadian Senate Special Committee on Illegal Drugs | ||||
| Volume I - General Orientation |
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Chapter 8 - Driving Under the Influence of CannabisEpidemiological data
According to a number of the witnesses we heard, more than 40% of people whose driving abilities are impaired would drive under the influence of cannabis. Others have said that approximately 12% of accidents causing injury could be attributed to the use of cannabis. What do the studies reveal? Data on the frequency of driving under the influence of cannabis (whether on its own or together with other substances) are, for obvious reasons, difficult to obtain. First, for drivers involved in an accident, a positive breathalyzer test means most of the time that no other measurements are taken because a blood alcohol level above the legal limit is enough to take legal action. Second, the methods available to detect the presence of THC are intrusive (blood, urine), unlike the breathalyzer, and hence pose specific legal and ethical problems. Other forms of measurements, such as saliva samples, do not, for the time being, allow roadside detection. Lastly, in studies of all drivers, the consent of drivers is required to take a blood or urine sample, thus limiting the possibility of generalizing results. Nevertheless, we will summarize the main points of a number of studies conducted in recent years.
Studies not involving accidentsTwo types of studies were conducted: surveys of all drivers selected at random from the flow of traffic at various times of the day and week, and studies where it was presumed that the people were driving under the influence during police checks. The following table, drawn from the various data available from INSERM, summarizes these studies. Epidemiological studies on youthIn recent years, epidemiological studies on youth in the school environment have asked questions about the frequency of driving under the influence of psychoactive substances, cannabis in particular. In Ontario, the 2002 OSDUS study described in Chapter 6 shows that 19.3% of the students had driven their car one hour or less after having taken cannabis at least once in the past twelve months.[1][19] More interesting is that this compares with 15% who said they had taken their car less than an hour after one or two drinks. In Manitoba, the survey of youths in school reveals that almost 20% see nothing wrong in driving after taking cannabis.[2][20] Finally, the Cohen and Kaal study on long term consumers had shown that no less than 42% of the respondents in Amsterdam and 74% in San Francisco had driven their car under the influence of cannabis.[3][21]
Risk assessmentGiven the difficulties of conducting reliable epidemiological studies on driving under the influence of cannabis, a number of authors have analyzed the probability of responsibility and the risk ratio involved in the use of cannabis. These studies distinguish between drivers who are responsible for accidents and those who are not. The former are the subjects and the latter the control group. Comparisons are then made of their intoxication to various substances. Clearly, placing drivers into the two categories of responsible/not responsible may depend on an investigator’s perception of whether or not psychoactive substances are present. The following table, which is reproduced from the Ramaekers et al. report (2002) for the International Scientific Conference on Cannabis summarizes the results of various studies.[4][22] It should be pointed out that the probability of responsibility for drivers showing traces of cannabis (D9THC and/or D9THC–COOH, whether measured in blood or urine) is compared to the responsibility of drivers involved in an accident not testing positive to any substance (including alcohol). The risk ratio for drivers not testing positive to any substances is 1.0 and is used as a point of comparison to determine the statistical significance of observed change in the risk level of impaired drivers. When the reference value is above the statistical confidence level of 95%, the obvious conclusion is that the drug is 95% associated with an increased risk of responsibility.
The study findings show that cannabis alone does not increase the likelihood of responsibility in an accident. However, most of the studies used a measurement of THC-COOH, an inactive metabolite that can remain in urine for several days. When the authors separated out THC alone, the risk ratio was slightly higher, even though it did not reach the required level of significance. In addition, as the concentration of THC increases, the more the ratio increases, once again suggesting a dose-response relationship. Furthermore, the cannabis and alcohol combination significantly increases risk. Without being able to draw any definite conclusions, there are some signs that their effects are in synergy and not merely additive. Studies on injured drivers (Terhune (1982) and Hunter (1998)) have ratios somewhat higher than in the other studies on fatal accidents. According to Bates and Blakely (1999), the apparent reduction in the risk of a fatal accident stems from the fact that drivers under the influence of cannabis drive less dangerously, for example by reducing their speed.[5][23] To conclude, we are rather in agreement with INSERM concerning these studies:
[translation] The findings definitely confirm the significant risk of alcohol, but generally fail to demonstrate that there is an effect of cannabis alone on the risk of being responsible for a fatal accident or an accident involving serious injury. The methodological difficulties that make such a demonstration difficult contribute considerably to the absence of statistically indisputable results. Analyses of responsibility nevertheless suggest that the association between alcohol and cannabis increases the risk of being responsible for an accident, compared to drinking alone; however, this finding needs to be consolidated. Lastly, the most recent data tend to show that there is a risk of becoming responsible at heavy concentrations of D9THC. This involves using cannabis immediately before driving, and perhaps applies also to chronic users. [6][24]
In all, it was observed that the detection rates for the presence of cannabis varied between 1% and 5% when there was no presumption of impaired driving. However, the missing data, which likely resulted from refusals to supply a sample, made it impossible to draw clear conclusions. The studies with presumption of driving under the influence of drugs had clearly higher results: between 10 and 26%. These results do not necessarily reveal a much higher prevalence of driving under the influence of psychoactive substances, but rather a higher level of vigilance by the police. Indeed, as we shall see immediately, the prevalence of cannabis detection in fatal accidents is no higher in Norway (7.5%) than in other countries.
Studies where an accident was involvedIt is difficult to compare studies between countries because the detection methods, even in an accident context, varies widely from country to country. We wish to note once again that simply finding traces of cannabis in drivers involved in accidents is not necessarily a sign that its use was the cause of the accident. Nor does the absence of any screening result mean that no one was driving under the influence of cannabis. The following table, adapted from INSERM results, refers to a number of recent studies in Europe, America and Australia.
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