A recent study examined the effect of state medical marijuana laws (MMLs) on cannabis-impaired driving in the U.S. Data on fatal motor vehicle accidents were obtained from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System (FARS) between 1993 and 2014; data on approximately 1.2 million drivers were obtained. The primary dependent variable in this study was whether the driver tested positive for THC (the psychoactive ingredient in cannabis) or a related metabolite. The author reviewed state medical marijuana policies in all states and coded MMLs by their effective dates, whether home cultivation was permitted, whether medical marijuana dispensaries existed and, if so, whether they were licensed. Among states with no MMLs implemented during the study period, (n = 28), the author categorized them by whether marijuana was decriminalized, recreational marijuana legalized, or a THC per se law (a universal impaired driving statute making it illegal for anyone to drive with a blood THC concentration above a specified limit) was in place. Additional state-level measures included the presence of alcohol per se laws, ratio of police officers to residents, and proxies for background substance use and marijuana market characteristics. Driver-level variables included positive tests for alcohol and illicit substances other than marijuana. This study employed a difference-in-differences design, in which the effect of MMLs was estimated via comparing changes in outcomes pre- and post-MML implementation to respective changes in outcomes in states that did not implement MMLs during the study period. Multiple imputation by chained equations was used to handle missing data, which were substantial (a limitation of FARS data). Results indicated 8.8% of drivers in the sample tested positive for cannabis, compared to 6.1% for stimulants, 4.2% for narcotics, and about twenty-five percent who had blood alcohol content (BAC) levels above 0.08 g/dL. Twenty-eight percent of drivers involved in fatal crashes were driving in a decriminalized state at the time of the crash, followed by 16% in THC per se law states, 14% in medical marijuana states, and 0.2% in recreational marijuana states. Overall, state MMLs were not significantly associated with marijuana-involved fatal crashes; however, in states with licensed medical marijuana dispensaries, there was a significant increase in the odds of marijuana-involved driving (Odds ratio [OR] = 1.14, 95% confidence interval [CI]: 1.02, 1.29). Neither home cultivation nor unlicensed dispensaries were significantly associated with marijuana-involved driving. The author also estimated the average treatment effect on the treated (ATET), which corresponded to the effect among states that implemented an MML during the study period. The first policy contrast indicated that drivers in MML states with licensed dispensaries had, on average, a 0.014 greater probability (p<0.01) of testing positive for cannabis compared to the counterfactual condition of no state MML. The second policy contrast, which compared drivers in MML states with licensed dispensaries to the counterfactual condition of MML states without licensed or unlicensed dispensaries, indicated a .011 greater probability (p < 0.05) of testing positive for cannabis. The author interpreted this to mean the presence of licensed dispensaries led to roughly 100 more drivers testing positive for marijuana than otherwise would have. A major limitation of this study is that the testing positive for marijuana does not necessarily mean a driver was impaired at the time, as marijuana metabolites can linger in the body long after impairment recedes.

Take away: This study documented that the presence of state medical marijuana laws in general were not associated with rates of positive screens for marijuana among drivers involved in fatal car crashes between 1993 and 2014. However, drivers in states that permitted licensed marijuana dispensaries had significantly higher odds of testing positive for marijuana than drivers in states without licensed dispensaries.

Sevigny, E. (2018). The effects of medical marijuana laws on cannabis-involved driving. Accident Analysis and Prevention [published online ahead of print June 7, 2018] doi: 10.1016/j.aap.2018.05.023