Cannabis impaired divided attention during driving in a simulator study
In a driving simulator study, each increase in blood THC impaired divided-attention performance, including task completion, accuracy, and lane-keeping during secondary tasks.
Quick Facts
What This Study Found
In a randomized, placebo-controlled crossover driving simulator study, each 1 ug/L increase in blood THC predicted increased odds of failing to complete a console search task (OR 1.05), more incorrect responses (OR 1.05), speed declines during mirror tasks, and longer lane departures. Low alcohol (~0.05% BrAC) separately worsened lane-keeping during secondary tasks.
Key Numbers
Each 1 ug/L blood THC: OR 1.05 for task failure, OR 1.05 for errors, 0.74% longer lane departures; BrAC ~0.05%: 1.41% longer lane departures during mirror task.
How They Did This
Randomized, placebo-controlled, crossover study with 6 sessions per participant: combinations of cannabis (placebo/low/high THC) and alcohol (placebo/active). Driving in full-motion NADS-1 simulator with three divided-attention tasks. Blood THC and breath alcohol measured.
Why This Research Matters
This is one of the most rigorous driving simulator studies using actual cannabis administration and a validated full-motion simulator. The dose-response findings directly inform impaired driving policy.
The Bigger Picture
The study provides concrete evidence that THC impairs the kind of multitasking drivers do constantly (checking mirrors, reading navigation, adjusting music) in a dose-dependent manner. Combined with even low alcohol levels, the impairment compounds.
What This Study Doesn't Tell Us
Simulator study (not real-world driving); controlled dosing may not reflect typical use patterns; participants were experienced cannabis users; specific tasks may not capture all real-world divided attention scenarios.
Questions This Raises
- ?Is there a blood THC threshold below which divided attention is not affected?
- ?How long after cannabis use does divided attention return to normal?
Trust & Context
- Key Stat:
- Each 1 ug/L blood THC: 5% higher odds of task failure and errors
- Evidence Grade:
- Strong: randomized, placebo-controlled, crossover design with full-motion driving simulator and pharmacokinetic monitoring.
- Study Age:
- Published 2020.
- Original Title:
- Impact of cannabis and low alcohol concentration on divided attention tasks during driving.
- Published In:
- Traffic injury prevention, 21(sup1), S123-S129 (2020)
- Authors:
- Miller, Ryan E, Brown, Timothy L(6), Lee, Stella, Tibrewal, Ishaan, Gaffney, Gary G, Milavetz, Gary, Hartman, Rebecca L, Gorelick, David A, Compton, Richard, Huestis, Marilyn A
- Database ID:
- RTHC-02725
Evidence Hierarchy
Participants are randomly assigned to treatment or placebo groups to test cause and effect.
What do these levels mean? →Frequently Asked Questions
Does cannabis affect multitasking while driving?
Yes. Each increase in blood THC predicted worse performance on three types of divided-attention tasks during simulated driving, including task completion, accuracy, and lane-keeping.
Does low alcohol make it worse?
Even at ~0.05% breath alcohol (below legal limits in most jurisdictions), alcohol added to lane-keeping impairment during secondary tasks, though it did not significantly affect task performance itself.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-02725APA
Miller, Ryan E; Brown, Timothy L; Lee, Stella; Tibrewal, Ishaan; Gaffney, Gary G; Milavetz, Gary; Hartman, Rebecca L; Gorelick, David A; Compton, Richard; Huestis, Marilyn A. (2020). Impact of cannabis and low alcohol concentration on divided attention tasks during driving.. Traffic injury prevention, 21(sup1), S123-S129. https://doi.org/10.1080/15389588.2020.1814956
MLA
Miller, Ryan E, et al. "Impact of cannabis and low alcohol concentration on divided attention tasks during driving.." Traffic injury prevention, 2020. https://doi.org/10.1080/15389588.2020.1814956
RethinkTHC
RethinkTHC Research Database. "Impact of cannabis and low alcohol concentration on divided ..." RTHC-02725. Retrieved from https://rethinkthc.com/research/miller-2020-impact-of-cannabis-and
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.