Marijuana use and driving in Washington state: Risk perceptions and behaviors before and after implementation of retail sales

March 2019

Objective: Washington is among the first states to legalize recreational use of marijuana. This study examined marijuana and alcohol use and risk perceptions before and after retail sales of recreational marijuana began in July 2014. A secondary objective was to examine the relationship between self-reported marijuana use and drug test results. Methods: Roadside surveys were conducted in three waves: June 2014, the month before retail sales of marijuana began; 5–6 months later (November–December 2014); and 1 year later (June 2015). A total of 2,355 drivers completed a marijuana questionnaire about their past and current marijuana use and perceived risks associated with driving after using marijuana. Data collection also included biological specimens (oral fluid and/or blood for marijuana testing and breath for alcohol testing). Drivers who tested positive for delta-9-tetrahydrocannabinol or 11-hydroxy-delta-9-tetrahydrocannabinol in oral fluid or blood were defined as THC-positive. Results: The proportion of drivers who reported past-year marijuana use was similar across the three surveys (fewer than one-third). Drivers’ perceived risk of impairment by marijuana and perceived risk of being arrested for marijuana-impaired driving were similar before and after retail sales. During daytime, the proportion of THC-positive drivers increased from 8% before retail sales to 23% 6 months after retail sales, but this proportion did not change among nighttime drivers (19% and 20%). Among THC-positive drivers, only 30%–35% reported past-year marijuana use during the first two surveys, but 72% reported past-year marijuana use 1 year after retail sales were legal. Conclusions: The prevalence of daytime THC-positive drivers increased substantially a few months after retail sales of marijuana were legal. Daytime and nighttime prevalence of THC-positive drivers was similar after retail sales. This pattern differs from that typically found for alcohol use, which is consistently higher among drivers at nighttime, compared with daytime. Reports of marijuana use were not always consistent with drug test results, which suggests that comparisons of self-reported marijuana use before and after legalization could be biased. This study examined marijuana use and risk perceptions over the course of 1 year. However, law changes may influence cultural norms gradually over a longer period of time. Future studies should continue to monitor marijuana use over time, as well as identify ways to determine whether drivers are impaired by marijuana.