Before raising the alarm on cannabis-impaired motor vehicle accidents, the stats presented in this article require additional clarification and contextualization.
Unlike alcohol, for which there is a close correlation between blood-alcohol content and impairment, there is no defined standard of impairment for cannabis.
Determining whether or not these drivers were impaired by cannabis is further complicated by the fact that tetrahydrocannabinol (THC) — the primary psychoactive component in cannabis — can be detected in the blood several days and even weeks after consuming cannabis.
This means that the observed increases in the proportion of fatally injured drivers testing positive for cannabis in these states might not indicate increases in cannabis-impaired driving, but rather general increases in cannabis use following legalization.
This is particularly plausible when we take into consideration the overall rate of motor vehicle accidents over time. Surveillance for cannabis among drivers in Colorado and Washington likely increased with legalization.