Impact of cannabis and low alcohol concentration on divided attention tasks during driving

Abstract

Objective

To assess divided-attention performance when driving under the influence of cannabis with and without alcohol. Three divided-attention tasks were performed following administration of placebo, cannabis, and/or alcohol.

Methods

Healthy adult cannabis users participated in 6 sessions, receiving combinations of cannabis (placebo/low-THC/high-THC) and alcohol (placebo/active) in randomized order, separated by washout periods of ≥1 week. At 0.5 hours post-dosing, participants performed simulator drives in the University of Iowa National Advanced Driving Simulator (NADS-1), a full vehicle cab simulator with a 360° horizontal field of view and motion base that provides realistic feedback. Drives contained repeated instances of three tasks: a side-mirror task (reaction to a triangle appearing in the side-mirrors), an artist-search task (select a specified artist from a navigable menu on the vehicle’s console), and a message-reading task (read aloud a message displayed on the console). Blood THC and breath alcohol concentration (BrAC) were interpolated using individual power curves from samples collected approximately 0.17, 0.42, 1.4, and 2.3 hours post-dose. Driving measures during tasks were compared to equal-duration control periods occurring just prior to the task. Performance shifts, task completion, and lane departures were modeled relative to blood THC and BrAC using mixed-effects regression models.

Results

Each 1 µg/L increase in blood THC concentration predicted increased odds of failing to complete the artist-search task (OR: 1.05, 95% CI: 1.01-1.11, p = 0.046), increased odds of selecting at least one incorrect response (OR: 1.05, 95% CI: 1.00-1.09, p = 0.041), declines in speed during the side-mirror task (0.005 m/s, 95% CI: 0.001-0.009, p = 0.023), and longer lane departure durations during the artist-search task (0.74% of task-period, 95% CI: 0.12-1.36 p = 0.020). BrAC (approximately 0.05%) was not associated with task performance, though each 0.01 g/210 L increase predicted longer departure durations during the side-mirror task (1.41% of task-period, 95% CI: 0.08-2.76, p = 0.040) and increased standard deviation of lane position in the message-reading task (0.61 cm, 95% CI: 0.14-1.08, p = 0.011).

Conclusions

With increasing medical and legal cannabis use, understanding the impact of acute cannabis use on driving performance, including divided-attention, is essential. These data indicate that impaired divided-attention performance is a safety concern.

Keywords:

• Cannabis
• THC
• driving
• alcohol
• divided-attention
• marijuana

Acknowledgments

Cannabis used in this study was obtained from the NIDA Drug Supply Program.

Data availability statement

The data that support the findings of this study are available from the corresponding author, RM, upon reasonable request.

Additional information

Funding

R.L. Hartman, D.A. Gorelick, and M.A. Huestis, Intramural Research Program, NIDA, NIH and interagency agreements between NIDA and the US Office of National Drug Control Policy and the National Highway Traffic Safety Administration; materials transfer agreements between NIDA and Storz & Bickel, who provided Volcano vaporizer devices/equipment for this study. T.L. Brown, G. Milavetz, A. Spurgin, and G. Gaffney, contract between National Highway Traffic Safety Administration and the University of Iowa (DTNH22-11-D-00237/004).