Role of alcohol and marijuana use in the initiation of fatal two-vehicle crashes.

PURPOSE: To assess individual and joint effects of alcohol and marijuana on the initiation of fatal two-vehicle crashes.

METHODS: Data on 14,742 culpable drivers (initiators) and 14,742 nonculpable drivers (noninitiators) involved in the same fatal two-vehicle crashes between 1993 and 2014 were obtained from the Fatality Analysis Reporting System. Multivariable conditional logistic regression models were used to assess the association of driver use of alcohol, marijuana, or both with fatal crash initiation with adjustment for demographic variables.

RESULTS: Initiators were significantly more likely than non-initiators to test positive for alcohol (28.3% vs. 9.6%, P < .0001), marijuana (10.4% vs. 6.0%, P < .0001), and both substances (4.4% vs. 1.1%, P < .0001). Relative to drivers testing negative for both alcohol and marijuana, the adjusted odds ratios of fatal crash initiation were 5.37 (95% confidence interval [CI]: 4.88 to 5.92) for those testing positive for alcohol and negative for marijuana, 1.62 (95% CI: 1.43 to 1.84) for those testing positive for marijuana and negative for alcohol, and 6.39 (95% CI: 5.19 to 7.88) for those testing positive for both alcohol and marijuana.

CONCLUSIONS: Alcohol and marijuana each play a significant role in fatal crash initiation. When used in combination, alcohol and marijuana appear to have a positive interaction effect on the risk of fatal crash initiation on the additive scale.