The Effects of Cannabis and Alcohol on Driving Performance and Driver Behaviour: A Systematic Review and Meta-Analysis

Abstract

Cannabis is the most frequently used drug in the world, and it is commonly detected in fatal crashes. Epidemiological research indicates that cannabis is associated with an increase in crash risk, but the mechanisms underlying this association remain unclear. The objective of the current systematic review and meta-analysis is to provide insight into these mechanisms by synthesizing experimental research focused on the effects of cannabis on driving performance and behaviour. Additionally, the experimental literature focused on the effects of alcohol on driving performance and behaviour is synthesized for comparative purposes. The four key aims of this dissertation are to (1) quantify the magnitude of the effect of cannabis on driving performance and behaviour; (2) compare the influence of cannabis to that of alcohol; (3) assess the effect of the combination of cannabis and alcohol on driving performance and behaviour; and, (4) identify knowledge gaps and quality limitations in the extant literature to direct the conduct of high quality research in the future. Academic Search Complete, CINAHL, Embase, Scopus, MEDLINE, PsycINFO, SportDISCUS and TRID were systematically searched in May 2018. Driving performance and behaviour data from experimental driving studies involving healthy participants of any age and sex collected in driving simulator, closed-course and on-road studies involving cannabis and/or alcohol administration, published in any language, were eligible for inclusion. Of 120 eligible studies, 81 were ultimately included in the meta-analysis. Most notably, cannabis was associated with impaired lateral control and decreased driving speed. Alcohol was associated with a variety of driving performance decrements and increased driving speed. The combination of drugs was associated with greater driving performance decrements than either drug in isolation. Finally, indirect comparisons indicated that the effects of cannabis on experimental driving measures were generally similar to low blood alcohol concentrations. However, imprecision in effect size estimates limits interpretation, and more research in the area is needed. Future research directions and quality recommendations are identified and described to aid in this endeavour. Nonetheless, the meta-analysis indicates that cannabis, like alcohol, impairs driving, and the combination of the two drugs is more detrimental to driving performance than either in isolation.