Marijuana and drugged driving: Challenges in evaluating the impact of marijuana-related laws on crashes

Reducing drugged driving is a national priority, including encouraging states to enact drug per se laws making it illegal to operate a motor vehicle while having positive levels of impairing drugs. The evaluation of these laws however requires reliable longitudinal data on drugs and driving. This study evaluates different approaches and data available for evaluating the effect of laws related to marijuana use on drugged driving. We evaluated the current state of knowledge regarding driving while under the influence of marijuana and laws likely to affect marijuana related driving. The availability of data sources on marijuana related driving and their ability to evaluate trends in marijuana related driving was also assessed. Only 15 states have drug per se laws for all drivers (2 for drivers <21). Several recurrent surveys assess marijuana use but not in relation to driving, have not asked specifically about marijuana, or state-level data is not available. No longitudinal data on marijuana use among injured drivers (ED or hospitalized) is available. Preliminary analyses of the Fatality Analysis Reporting System (FARS) found most states do not consistently test fatally injured drivers for drugs, especially THC; historical data on marijuana is inconsistent. Among 15 states with drug per se laws only five drug tested more than 75% of driver fatalities and only three had longitudinal data on THC as did four comparison states. The lack of reliable longitudinal data on drugs and driving is a major impediment to evaluating marijuana laws.