Simultaneous Alcohol and Marijuana Use As a Predictor of Risky Driving Behaviors Among Young Adults

Introduction: Driving while under the influence of alcohol and/or marijuana is a major health risk behavior among young adults and has been linked to reckless driving, motor vehicle accidents, and fatalities. In the US, 1/3 of deaths from motor vehicle accidents involved driving under the influence of alcohol and over the past two decades, accidents involving drivers under the influence of marijuana have increased. Young adults who engage in simultaneous alcohol and marijuana (SAM) use are more likely to report risky driving behaviors. Some young adults report a belief that SAM use can help to reduce the risks of driving while intoxicated. However, it is unclear whether people who engage in SAM use also are more likely to take driving risks, or whether occasions of SAM use are associated with increased risk (compared to other occasions of substance use for the same person). Clarifying this issue could help to inform the need for prevention programming.

Methods: Young adult SAM users (N = 378) reported twice per day on their substance use and driving behavior for up to 14 days each, yielding 1590 days of sole marijuana use, 1340 days of sole alcohol use, and 602 days of SAM use. Multilevel models were conducted predicted driving within 3 hours of substance use and riding as a passenger with an intoxicated driver as a function of whether participants reported SAM use, compared to using marijuana or alcohol alone, controlling for covariates.

Results: Models predicting driving behavior yielded no significant results for SAM use. However, young adults were significantly more likely to ride in a car with an intoxicated driver on days that they engaged in SAM use compared to days that they only used marijuana (OR = 1.86, p < .01).

Conclusions: Although these results do not indicate that young adults are more likely to drive after SAM use, they suggest that SAM use may have a relatively stronger impairing effect on judgment than marijuana, leading young adults to decide to ride with an intoxicated driver. It may also suggest that situations in which young adults engage in SAM use (e.g., parties) are more likely to lead to driving with an intoxicated driver than situations in which they use marijuana alone. This indicates the need for future research that explores motivations for driving with an intoxicated driver as a function of the types of substances used.