Abstract
BackgroundConcurrent use of marijuana and alcohol in drivers is of increasing concern but its role in crash causation has not been well understood.Methods Using a case–control design, we assessed the individual and joint effects of marijuana and alcohol use on fatal crash risk. Cases (n = 1944) were drivers fatally injured in motor vehicle crashes in the United States at specific times in 2006, 2007 and 2008. Controls (n = 7719) were drivers who participated in the 2007 National Roadside Survey of Alcohol and Drug Use by Drivers.ResultsOverall, cases were significantly more likely than controls to test positive for marijuana (12.2% vs. 5.9%, p < 0.0001), alcohol (57.8% vs. 7.7%, p < 0.0001) and both marijuana and alcohol (8.9% vs. 0.8%, p < 0.0001). Compared to drivers testing negative for alcohol and marijuana, the adjusted odds ratios of fatal crash involvement were 16.33 [95% confidence interval (CI): 14.23, 18.75] for those testing positive for alcohol and negative for marijuana, 1.54 (95% CI: 1.16, 2.03) for those testing positive for marijuana and negative for alcohol, and 25.09 (95% CI: 17.97, 35.03) for those testing positive for both alcohol and marijuana.ConclusionsAlcohol use and marijuana use are each associated with significantly increased risks of fatal crash involvement. When alcohol and marijuana are used together, there exists a positive synergistic effect on fatal crash risk on the additive scale.
Highlights
Concurrent use of marijuana and alcohol in drivers is of increasing concern but its role in crash causation has not been well understood
12.2% of the 1944 cases and 5.9% of the 7719 controls tested positive for marijuana, yielding a crude Odds ratio (OR) of 2.21
The risk of fatal crash involvement increased with Blood alcohol concentration (BAC) levels from a crude OR of 2.65 for 0.01–0.07 g/dL to 63.14 for ≥0.08 g/dl (Table 1)
Summary
Concurrent use of marijuana and alcohol in drivers is of increasing concern but its role in crash causation has not been well understood. Driving under the influence of drugs (DUID) is a serious public safety concern in the United States and around the world (Brady & Li 2013; Romano & Voas 2011; Kaplan et al 2006; Berning et al 2015; Hartman & Huestis 2013; Walsh et al 2005; Dubois et al 2015). Since 1996, 28 states and Washington, DC have enacted legislation to decriminalize marijuana for medical use (NCSL 2016). Colorado has registered an increase in fatal motor vehicle crashes involving marijuana since legalizing marijuana for medical use in 2000 (Urfer et al 2014) compared to insignificant changes in states without medical marijuana laws (Salomonsen-sautel et al 2014)
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