AAA Foundation for Traffic Safety, January 2020
Washington State Initiative 502 (I-502), effective Dec. 6, 2012, legalized possession of small amounts of cannabis for recreational use by adults aged 21 years and older. It also included a prohibition against driving with 5 or more nanograms of delta-9-tetrahydrocannabinol (THC) per milliliter of blood, along with a zero tolerance prohibition for drivers younger than 21 years of age. THC is the main psychoactive component in cannabis and detection of THC in blood is suggestive of recent use. A previous study by the AAA Foundation for Traffic Safety examined data from drivers involved in fatal crashes in Washington State in years 2010-2014 and estimated that the proportion of drivers with detectable THC approximately doubled several months after I-502 became effective (Tefft et al., 2016). The research reported here updates the previous study with three additional years of data, post-legalization. Multiple imputation was used to estimate the proportion of drivers who were THC-positive among those who were not tested for drugs or whose test results were unavailable. Results indicate that five years after I-502, the proportion of fatal-crash-involved drivers who are THC-positive has remained approximately double the level observed before I-502. An estimated 21% of all drivers involved in fatal crashes in Washington State in 2017 were THC-positive, higher than in any other year in the 10-year period examined.
