Cannabis Impaired Simulated Driving More Than Alcohol at the Doses Tested, Regardless of Experience
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- Cannabis decreased speed and increased variability; alcohol slightly increased speed, with different impairment profiles
- Evidence Grade:
- Well-designed within-subject comparison with multiple dose conditions and two experience levels. Simulator limitations and moderate alcohol doses are caveats.
- Study Age:
- Published in 2010. Cannabis driving impairment research has continued to expand, though simulator studies still cannot fully replicate real-world driving conditions.
- Original Title:
- The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand.
- Published In:
- Accident; analysis and prevention, 42(3), 859-66 (2010)
- Authors:
- Lenné, Michael G, Dietze, Paul M, Triggs, Thomas J, Walmsley, Susan, Murphy, Brendan, Redman, Jennifer R
- Database ID:
- RTHC-00427
Evidence Hierarchy
Participants are randomly assigned to treatment or placebo groups to test cause and effect.
What do these levels mean? →Frequently Asked Questions
Is cannabis or alcohol more dangerous for driving?
This study found cannabis impaired more driving measures at the tested doses. However, the alcohol doses were moderate, and epidemiological evidence consistently shows alcohol as a greater crash risk factor. The impairment profiles differ: cannabis affects automatic functions while alcohol affects decision-making.
Why did combining them not make things worse?
The alcohol doses may have been too low to produce significant impairment on their own, limiting the potential for synergistic effects. Other studies using higher alcohol doses have found that the combination is more dangerous than either drug alone.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-00427APA
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
MLA
Lenné, Michael G, et al. "The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand.." Accident; analysis and prevention, 2010. https://doi.org/10.1016/j.aap.2009.04.021
RethinkTHC
RethinkTHC Research Database. "The effects of cannabis and alcohol on simulated arterial dr..." RTHC-00427. Retrieved from https://rethinkthc.com/research/lenne-2010-the-effects-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.