In a driving simulator study, each increase in blood THC impaired divided-attention performance, including task completion, accuracy, and lane-keeping during secondary tasks.
Traffic safety researchers, impaired driving policy makers, and cannabis users who drive.
Each 1 ug/L blood THC: 5% higher odds of task failure and errors
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
Strong: randomized, placebo-controlled, crossover design with full-motion driving simulator and pharmacokinetic monitoring.
When this study was published
Published 2020.
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.
Questions still open
- Is there a blood THC threshold below which divided attention is not affected? How long after cannabis use does divided attention return to normal?
Common questions
Does cannabis affect multitasking while driving?
Does low alcohol make it worse?
Read the original research
Impact of cannabis and low alcohol concentration on divided attention tasks during driving.
Traffic injury prevention, 21(sup1), S123-S129
Citation
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