Cannabis effects on driving lateral control with and without alcohol

Abstract

Background: Effects of cannabis, the most commonly encountered non-alcohol drug in driving under the influence cases, are heavily debated. We aim to determine how blood Δ(9)-tetrahydrocannabinol (THC) concentrations relate to driving impairment, with and without alcohol.

Methods: Current occasional (≥1×/last 3 months, ≤3days/week) cannabis smokers drank placebo or low-dose alcohol, and inhaled 500mg placebo, low (2.9%)-THC, or high (6.7%)-THC vaporized cannabis over 10min ad libitum in separate sessions (within-subject design, 6 conditions). Participants drove (National Advanced Driving Simulator, University of Iowa) simulated drives (∼0.8h duration). Blood, oral fluid (OF), and breath alcohol samples were collected before (0.17h, 0.42h) and after (1.4h, 2.3h) driving that occurred 0.5-1.3h after inhalation. We evaluated standard deviations of lateral position (lane weave, SDLP) and steering angle, lane departures/min, and maximum lateral acceleration.

Results: In N=18 completers (13 men, ages 21-37years), cannabis and alcohol increased SDLP. Blood THC concentrations of 8.2 and 13.1μg/L during driving increased SDLP similar to 0.05 and 0.08g/210L breath alcohol concentrations, the most common legal alcohol limits. Cannabis-alcohol SDLP effects were additive rather than synergistic, with 5μg/L THC+0.05g/210L alcohol showing similar SDLP to 0.08g/210L alcohol alone. Only alcohol increased lateral acceleration and the less-sensitive lane departures/min parameters. OF effectively documented cannabis exposure, although with greater THC concentration variability than paired blood samples.

Conclusions: SDLP was a sensitive cannabis-related lateral control impairment measure. During drive blood THC ≥8.2μg/L increased SDLP similar to notably-impairing alcohol concentrations. Despite OF's screening value, OF variability poses challenges in concentration-based effects interpretation.

Keywords: Alcohol; Cannabis; Driving; Lateral control; Oral fluid; THC.

Conflict of interest statement

Conflicts of Interest

Volcano® and Quantisal™ devices and supplies (Storz & Bickel, Tuttlingen, Germany and Immunalysis, Pomona, CA) were provided by manufacturers through Materials Transfer Agreements. No commercial entity played any role in study design and conduct, data analysis, manuscript drafting, or the decision to publish. The authors declare no personal conflicts of interest.

Published by Elsevier Ireland Ltd.

Figures

Figure 1

The National Advanced Driving Simulator: A) exterior, dome mounted in room; B) dome interior with car mounted inside; C) view of night-drive simulation.

Figure 2

GLM Select modeled standard deviation of lateral position (SDLP) versus blood Δ9-tetrahydrocannabinol (THC) concentration (lower x-axis) and versus breath alcohol concentration (BrAC, upper x-axis). Note x-axis scales are different so slopes cannot be directly compared; dotted lines indicate THC concentrations producing equivalent SDLP to 0.02, 0.05, and 0.08g/210L BrAC.

Figure 3

Box plot of maximum blood Δ9-tetrahydrocannabinol (THC) concentration by administered cannabis (placebo, 0.008% THC; low, 2.9% THC; high, 6.7% THC) and alcohol (placebo, active) doses for 18 participants.