Grant: Project Grant

Background: THC, the main impairing ingredient in cannabis, impairs driving performance and increases the risk of motor vehicle collisions. Many countries make it illegal to drive with THC above a certain legal limit. Legal limits simplify enforcement of traffic laws and can be effective tools for preventing impaired driving. However, evidence supporting current legal limits for THC is limited. There is poor correlation between THC level and impairment, especially in frequent cannabis users who develop tolerance. In addition, current legal limits ignore 11-OH-THC, a break-down product of THC that forms after ingesting edibles and also causes impairment. This suggests that THC level alone may not be closely related to collision risk and other factors such as indicators of recent use should be considered. Aims: This project has four aims. First, we will investigate whether drivers with evidence of recent cannabis use have increased risk of collisions. Second, we will study whether the risk of crashing associated with THC is different in drivers with evidence of frequent cannabis use. We hypothesize that these drivers will have tolerance to THC and their risk, at any THC level, will be less. Third, we will study whether drivers with high levels of 11-OH-THC are more likely to have caused the crash. Finally, we will provide updates on the numbers of injured drivers in British Columbia who test positive for cannabis, alcohol and/or other drugs. Methods: This research will build on the Cannabis and Motor Vehicle Crashes (CMVC) study which has measured drug and alcohol levels and obtained police reports of injured BC drivers since 2011. Police reports are reviewed to determine which drivers caused the collision. In our analysis, we compare drug detection rates in drivers who caused the crash with drivers who were "innocently involved" in a collision.