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.
Read this if you want to understand how cannabis driving impairment differs from alcohol impairment.
Cannabis users compensated by driving slower; combining with alcohol eliminated this ability
What the researchers 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.
Why it 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 numbers in context
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 the study worked
Narrative review of experimental and epidemiological research comparing cannabis and alcohol effects on driving-related skills, on-road driving performance, and crash risk.
What this study cannot 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.
How to read the evidence
Narrative review synthesizing experimental and epidemiological evidence. Provides a useful framework but did not systematically assess study quality.
When this study was published
Published in 2009. Cannabis-impaired driving research has continued to expand, with most subsequent studies supporting the general findings described here.
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.
Questions still open
- 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?
Common questions
Is it safe to drive after using cannabis?
Why is combining cannabis and alcohol so dangerous?
Read the original research
The effect of cannabis compared with alcohol on driving.
The American journal on addictions, 18(3), 185-93
Citation
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