Review of the literature on cannabis and crash risk

Abstract

A review of the literature published prior to 2005 concerning cannabis and road crash involvement was conducted, with emphasis given to studies documenting the relative crash risk associated with driving after use of cannabis. Case-control studies that have been conducted into cannabis and road crashes have been characterised by methodological flaws that make the interpretation of the results difficult. Partly as a response to the difficulty of conducting case-control studies, some researchers have used culpability studies to determine whether cannabis use contributes to crash involvement. However, as for case-control studies into cannabis and crash involvement, many culpability studies are difficult to interpret because of methodological problems. There have been two recent Australian studies that have analysed the relationship between THC (tetrahydrocannabinol - the psychoactive component of cannabis) measured in the blood and crash culpability. These two studies produced contradictory results. In summary, the risk of crash involvement associated with driving under the influence of cannabis remains to be determined. To resolve the issue, it is necessary to conduct a case-control study similar to those that have been conducted for alcohol. That is, it is necessary to compare the incidence of cannabis in crash-involved drivers with the incidence in non-crash-involved drivers matched for potential confounding factors, such as age, gender, time of day, day of week, and direction of travel.