In a driving simulator, high-dose cannabis impaired multiple driving measures in both novice and experienced drivers, while alcohol at the tested doses had fewer effects and did not produce synergistic impairment when combined.
Read this if you want to understand how cannabis and alcohol differently affect driving ability.
Cannabis decreased speed and increased variability; alcohol slightly increased speed, with different impairment profiles
What the researchers found
Twenty-five experienced and 22 inexperienced drivers completed nine simulated driving conditions combining three cannabis doses (placebo, low, high) with three alcohol doses (placebo, low, high).
High cannabis doses caused decreased mean speed, increased speed and lateral position variability, increased headways, and longer reaction times. Cannabis impairment was greater at higher doses.
Alcohol at the tested doses (approximately 0.4 and 0.6 g/kg) had relatively few effects and was associated with a slight increase in speed (opposite to cannabis, which decreased speed).
Surprisingly, combining cannabis and alcohol did not produce synergistic effects beyond what each drug produced alone. Both substances increased speed and lateral position variability.
The driving environment included varied workload conditions and a secondary task to test demand effects.
Why it matters
This study directly compared cannabis and alcohol effects on driving in the same participants, providing a within-subject comparison. The finding that cannabis slowed drivers while alcohol slightly sped them up illustrates fundamentally different impairment profiles.
The numbers in context
25 experienced, 22 inexperienced drivers. 9 conditions. Cannabis: 19 mg THC cigarettes. Alcohol: 0, 0.4, 0.6 g/kg. High cannabis: decreased speed, increased variability, longer reaction time. Low-moderate alcohol: few effects.
How the study worked
Counterbalanced nine-condition study (3 cannabis x 3 alcohol doses) in 25 experienced and 22 inexperienced drivers using a driving simulator with arterial road environment. Cannabis: pre-rolled cigarettes with 19 mg THC. Alcohol: approximately 0, 0.4, 0.6 g/kg.
What this study cannot tell us
Driving simulator does not fully represent real-world driving. Alcohol doses were relatively low (0.4-0.6 g/kg) and may not have reached impairment-producing levels. Cannabis was a single high dose (19 mg). The absence of synergy at these doses does not rule out synergy at higher doses.
How to read the evidence
Well-designed within-subject comparison with multiple dose conditions and two experience levels. Simulator limitations and moderate alcohol doses are caveats.
When this study was published
Published in 2010. Cannabis driving impairment research has continued to expand, though simulator studies still cannot fully replicate real-world driving conditions.
The bigger picture
These findings contrast with some earlier studies that found cannabis-alcohol synergy. The relatively low alcohol doses used may explain the lack of synergistic effects, though the qualitative differences between cannabis and alcohol impairment were clearly demonstrated.
Questions still open
- Would higher alcohol doses reveal synergistic effects with cannabis? Do the different impairment profiles suggest different accident types for cannabis vs alcohol? Should novice drivers be treated differently in cannabis driving policy?
Common questions
Is cannabis or alcohol more dangerous for driving?
Why did combining them not make things worse?
Read the original research
The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand.
Accident; analysis and prevention, 42(3), 859-66
Citation
Lenné, Michael G; Dietze, Paul M; Triggs, Thomas J; Walmsley, Susan; Murphy, Brendan; Redman, Jennifer R. (2010). The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand.. Accident; analysis and prevention, 42(3), 859-66. https://doi.org/10.1016/j.aap.2009.04.021