THE EFFECTS OF CANNABIS AND THE LEGALIZATION OF MARIJUANA ON FATAL CRASHES IN WASHINGTON STATE

Abstract

As cannabis use is legalized in more states, traffic crashes and fatalities related to cannabis and other drugs are becoming a major public health and safety issue. While researchers and policy makers have focused considerable attention on the impact of legalization initiatives (such as I-502) on public health and safety, much remains unknown about the effects of cannabis intoxication and legalization initiatives on the risk of a motor vehicle crash. Specifically, the sole effect of delta-9-tetrahydrocannabinol (THC), which is the main psychoactive chemical in marijuana, and the possible interaction effect of THC with alcohol on motor vehicle crashes are unclear. The current study attempted to fill this gap by examining the links between THC, alcohol, their interaction, and undesirable outcomes involved in fatal crashes using Washington State Fatality Analysis Reporting System (WA FARS) data from 2008 to 2017. Findings from bivariate correlation analyses showed a number of significant relationships between THC intoxication and individual-level variables, including age, gender, unlicensed driver, and driver history of traffic violations. In addition, results from binary logistic analyses showed that the presence of THC was not a significant predictor of speeding and driver error. Moreover, findings from three additional analyses, including PSM, PSW, and qualitative file review, showed that drivers who had a positive blood test for alcohol alone or the combination of THC and alcohol were more likely to engage in risky behaviors and be exposed to undesirable outcomes in fatal crashes. However, THC alone was not a risk factor for undesirable outcomes related to fatal crashes. Particularly, the results for THC alone seemed to vary and the effect size was low. Finally, results from an interrupted time-series analysis showed a statistically significant increase in THC-positive drivers involved in fatal crashes after the implementation of I-502 in Washington, but showed no further increase after cannabis retail shops opened in July 2014. Overall, a strong and consistent effect of THC in the blood of drivers on adverse outcomes involved in fatal crashes was not found in the current study.