Among U.S. high school seniors, simultaneous use of alcohol and marijuana was associated with the highest rates of unsafe driving, exceeding the risk from using either substance alone.
Read this if you want to understand how combined alcohol and marijuana use affects teen driving safety.
Simultaneous alcohol + marijuana use had the highest unsafe driving rates across 35 years of data
What the researchers found
Analysis of 72,053 high school seniors surveyed from 1976 to 2011 found that higher substance use frequency, particularly alcohol use frequency, was significantly associated with unsafe driving (tickets, warnings, or accidents).
Simultaneous use (using alcohol and marijuana at the same time) was associated with the highest rates of unsafe driving, followed by concurrent use (using both substances but at different times), followed by alcohol use alone. Individuals who reported simultaneous use "most or every time" they used marijuana had the highest likelihood of reporting unsafe driving after either substance.
The risk gradient was clear: same-time combined use > different-time combined use > single substance use, suggesting the pharmacological interaction between alcohol and marijuana compounds impairment.
Why it matters
The distinction between simultaneous and concurrent use is rarely examined but appears to be important for driving risk. Prevention messaging that addresses the specific danger of combining alcohol and marijuana at the same time could help reduce teen driving fatalities.
The numbers in context
72,053 students surveyed (1976-2011). Risk hierarchy: simultaneous use > concurrent use > alcohol alone > marijuana alone. Simultaneous users had highest rates of driving-related tickets, warnings, and accidents.
How the study worked
Analyses used data from 72,053 students collected through Monitoring the Future annual surveys of nationally representative cross-sectional samples of U.S. 12th-grade students from 1976 to 2011. Two aspects of substance use were examined: frequency and simultaneous versus concurrent versus single-substance use status. Unsafe driving measures included tickets/warnings and accidents.
What this study cannot tell us
All data was self-reported, including both substance use and driving outcomes. The cross-sectional design cannot establish causation. The study does not measure actual impairment, only self-reported unsafe driving events. Cannabis potency has changed dramatically over the study period (1976-2011), which could affect the relationship.
How to read the evidence
This is a very large nationally representative dataset spanning decades, providing robust evidence for the association between simultaneous use and unsafe driving.
When this study was published
Published in 2014 with data through 2011. The landscape of teen substance use and driving has continued to evolve with legalization.
The bigger picture
As marijuana legalization expands, understanding the combined effects of alcohol and marijuana on driving is increasingly important. This large, nationally representative dataset provides strong evidence that simultaneous use poses a substantially greater risk than either substance alone.
Questions still open
- What are the pharmacological mechanisms by which simultaneous alcohol and marijuana use compound driving impairment? Would targeted prevention messaging about simultaneous use be more effective than general substance use prevention? Have these patterns changed since marijuana legalization?
Common questions
What is the difference between simultaneous and concurrent use?
Why is simultaneous use more dangerous for driving?
Read the original research
Alcohol and marijuana use patterns associated with unsafe driving among U.S. high school seniors: high use frequency, concurrent use, and simultaneous use.
Journal of studies on alcohol and drugs, 75(3), 378-89
Citation
Terry-McElrath, Yvonne M; O'Malley, Patrick M; Johnston, Lloyd D. (2014). Alcohol and marijuana use patterns associated with unsafe driving among U.S. high school seniors: high use frequency, concurrent use, and simultaneous use.. Journal of studies on alcohol and drugs, 75(3), 378-89.