How Cannabis-Impaired Driving Differs From Drunk Driving
Cannabis impaired automatic driving functions more than complex tasks and drivers tended to compensate, while alcohol showed the opposite pattern; combining both eliminated compensatory behavior and multiplied impairment.
Quick Facts
What This Study Found
This review compared the driving-related effects of cannabis and alcohol.
Both substances impaired driving skills in a dose-related fashion, but the patterns were fundamentally different. Cannabis impaired highly automatic driving functions (lane tracking, routine reactions) more than complex tasks requiring conscious control. Alcohol showed the opposite pattern, impairing complex decision-making more than automatic functions.
Critically, cannabis users tended to recognize their impairment and compensate by driving slower, increasing following distance, and using other behavioral strategies. This compensation did not occur with alcohol.
However, combining cannabis and alcohol eliminated the ability to compensate effectively, resulting in impairment even at individually insignificant doses of each drug.
Epidemiological studies were inconclusive about whether cannabis alone increased accident risk, while alcohol use unanimously increased crash risk.
Key Numbers
Drunk drivers involved in 25% of motor vehicle fatalities. Cannabis effects varied more between individuals than alcohol due to tolerance, smoking technique, and absorption differences. Combination eliminated compensatory driving.
How They Did This
Narrative review of experimental and epidemiological research comparing cannabis and alcohol effects on driving-related skills, on-road driving performance, and crash risk.
Why This Research Matters
Understanding how cannabis and alcohol differ in their driving effects is essential for drug-impaired driving policy, which has largely been modeled on alcohol impairment assumptions that may not apply to cannabis.
The Bigger Picture
Drug-impaired driving policy for cannabis cannot simply copy the alcohol model. The different impairment patterns, the role of tolerance, and the ability to compensate all create a more complex picture than exists for alcohol.
What This Study Doesn't Tell Us
The review included studies with varying methodologies and doses. Compensatory behavior in laboratory settings may not reflect real-world driving decisions. Epidemiological evidence was described as inconclusive.
Questions This Raises
- ?Should cannabis driving limits account for tolerance?
- ?How long after use is driving safe?
- ?Should per se THC limits be based on impairment studies rather than arbitrary thresholds?
- ?How can combined use be addressed in policy?
Trust & Context
- Key Stat:
- Cannabis users compensated by driving slower; combining with alcohol eliminated this ability
- Evidence Grade:
- Narrative review synthesizing experimental and epidemiological evidence. Provides a useful framework but did not systematically assess study quality.
- Study Age:
- Published in 2009. Cannabis-impaired driving research has continued to expand, with most subsequent studies supporting the general findings described here.
- Original Title:
- The effect of cannabis compared with alcohol on driving.
- Published In:
- The American journal on addictions, 18(3), 185-93 (2009)
- Authors:
- Sewell, R Andrew, Poling, James(2), Sofuoglu, Mehmet(8)
- Database ID:
- RTHC-00392
Evidence Hierarchy
Summarizes existing research on a topic.
What do these levels mean? →Frequently Asked Questions
Is it safe to drive after using cannabis?
Cannabis does impair driving skills, particularly automatic functions like lane tracking. While users tend to compensate more than drunk drivers, impairment is still present. The review recommended waiting several hours before driving and never combining with alcohol.
Why is combining cannabis and alcohol so dangerous?
Individually, cannabis users compensate for their impairment by driving more cautiously. Alcohol prevents this compensatory behavior. Together, the impairment effects add up while the protective compensatory strategies are eliminated.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-00392APA
Sewell, R Andrew; Poling, James; Sofuoglu, Mehmet. (2009). The effect of cannabis compared with alcohol on driving.. The American journal on addictions, 18(3), 185-93. https://doi.org/10.1080/10550490902786934
MLA
Sewell, R Andrew, et al. "The effect of cannabis compared with alcohol on driving.." The American journal on addictions, 2009. https://doi.org/10.1080/10550490902786934
RethinkTHC
RethinkTHC Research Database. "The effect of cannabis compared with alcohol on driving." RTHC-00392. Retrieved from https://rethinkthc.com/research/sewell-2009-the-effect-of-cannabis
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.