The purpose of this study was to quantify the prevalence of marijuana involvement in fatal crashes in the state of Washington in years 2010 – 2014 and to investigate whether the prevalence changed after Washington Initiative 502, which legalized recreational use of marijuana for adults aged 21 years and older and also created a new per se limit for driving under the influence of marijuana, took effect on 6 December 2012.

The data examined were obtained from the Washington Traffic Safety Commission and comprised a census of all motor vehicle crashes that occurred on public roads in the state of Washington and resulted in a death within 30 days. This study examined the presence and concentration of delta-9-tetrahydrocannabinol (hereafter THC), the main psychoactive chemical in marijuana, in the blood toxicological test results of drivers involved in fatal crashes. THC presence and concentration in the subset of drivers whose blood was not tested or whose test results were unavailable were estimated using the method of multiple imputation. The imputation method explicitly accounted for changes implemented during the study period in the cutoff levels used in the state laboratory for detection of THC.

Statewide, 3,031 drivers were involved in fatal crashes in years 2010 – 2014. Overall, considering both the actual blood toxicology test results and imputed results, an estimated 303 drivers—10.0% of all drivers involved in fatal crashes in Washington between 2010 and 2014—had detectable THC in their blood at or shortly after the time of the crash. Of all THC-positive drivers involved in fatal crashes, an estimated 34.0% had neither alcohol nor other drugs in their blood, 39.0% had detectable alcohol in addition to THC, 16.5% had other drugs in addition to THC, and 10.5% had had both alcohol and other drugs in addition
to THC in their blood.

From 2010 through 2013, the estimated number and proportion of drivers involved in fatal crashes who had a detectable concentration of THC in their blood ranged from a low of 48 (7.9%) to a high of 53 (8.5%); the number and proportion both approximately doubled from 49 (8.3%) in 2013 to 106 (17.0%) in 2014. Analysis of trends over time before and after Initiative 502 took effect indicated that the proportion of drivers positive for THC was generally flat before and immediately after Initiative 502 took effect, but began increasing significantly at a rate of 9.7 percentage points per year approximately 9 months after Initiative 502 took effect. It was not clear whether this increasing trend was attributable to Initiative 502 or to other factors that were beyond the scope of the study.

THC levels in blood fall rapidly shortly after cannabis consumption, thus, it is possible that some surviving drivers in fatal crashes may have had a detectable concentration of THC in their blood at the time of the crash but that their THC levels had fallen below the minimum detectable level by the time a blood sample was drawn. Also, results of this study do not indicate that drivers with detectable THC in their blood at the time of the crash were necessarily impaired by THC or that they were at-fault for the crash; the data available cannot be used to assess whether a given driver was actually impaired, and examination of fault in individual crashes was beyond the scope of this study.