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Acute Cannabis Consumption And Motor Vehicle Collision Risk: Systematic Review Of Observational Studies And Meta-Analysis

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Despite being regulated in many jurisdictions, cannabis (marijuana) is the most widely used illicit substance in the world. Results from the 2009 Canadian Alcohol and Drug Use Survey have indicated that 11.4% of Canadians overall and 33% of those aged 15-24 years used cannabis at least once in the previous year.1Rates of driving under the influence of cannabis have also risen in recent years; national data collected in 2004 indicate that 4% of Canadian adults reported driving within one hour of consuming cannabis, up from 1.9% recorded in 1996-7.2 These results are reflected in other jurisdictions across the world. A roadside survey of 537 drivers in Scotland reported that 15% of respondents aged 17-39 years admitted to having consumed cannabis within 12 hours of driving a vehicle,3 and the European Monitoring Centre for Drugs and Drug Addiction found that between 0.3% and 7.4% of drivers tested positive for cannabis from roadside surveys in the United Kingdom, Denmark, the Netherlands, Norway, the United States, and Australia.4


Much of the early research assessing the effects of cannabis on driving performance was done by laboratory and driving simulator studies. The results of these studies are generally consistent: at increased doses, cannabis impairs the psychomotor skills necessary for safe driving.5 6 7 8 9 10 11 12However, although laboratory studies have high internal validity with regard to the dose related effects of cannabis on performance, the dose-response association is unclear in relation to driving ability and collision risk outside the laboratory.7 8 13 14 As a result, these studies do not always translate well to driving scenarios in the real world, and generally focus on experienced cannabis users consuming the drug in unorthodox surroundings and undertaking tasks that do not always reflect the complex nature of driving in natural settings.15


Observational epidemiology studies can assess driving in the general population and are able to address many of the limitations of laboratory studies.16 Three types of epidemiological studies are commonly used to investigate cannabis use and motor vehicle collisions: cross sectional studies, cohort studies, and case-control studies. Many cross sectional studies have reported on cannabis incidence in injured or fatally injured drivers, as well as in the general driving public.17 18 19 20 21 22 23In these studies, cannabis is consistently one of the most frequently detected psychoactive substances (second after alcohol), and individuals who drive within two hours of using cannabis have raised rates of collision.24


Fewer case-control and cohort studies have looked at cannabis consumption and collision risk, and their results have been inconsistent.525 26 27 More than half of these studies have suggested that cannabis consumption is associated with an increased risk of traffic collision,21 23 28 and the remaining studies have found no association or a decreased risk of collision.29 30 31 32 33 Researchers have also used a variant of case-control designs, often known as culpability studies.34 Culpability studies include drivers involved in collisions, separated into those who were responsible for the collision and those who were not. The premise of these studies is that, if cannabis use increases collision risk, the drug should more likely be detected in drivers judged to be responsible for their collision. However, culpability analyses have also produced mixed results.35 36 37 38 39


Therefore, a lack of consensus exists on whether the risk of motor vehicle collisions is elevated or lowered when drivers have recently consumed cannabis. Furthermore, very few robust studies on this subject are generalisable to situations in the real world. An up to date systematic review is necessary to integrate the existing evidence on the role of cannabis use on collision risk, not only from a public policy and programme perspective, but also in view of the current gaps in scope and quality of literature and methodology.


We did a systematic review of the observational epidemiology literature to ascertain whether the acute consumption of cannabis or cannabinoids by drivers increases the risk of a motor vehicle collision, and to explore the impact of potential biases due to outcome measurement and confounding on the observed effect sizes.


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