A Simulator Study of the Combined Effects of Alcohol and Marihuana on Driving Behavior–Phase II

Stein, A. C.; Allen, R. W.; Cook, M. L.; Karl, R. L. · 1983 · ROSA P / United States. National Highway Traffic Safety Administration

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Summary

This study, conducted by the National Highway Traffic Safety Administration (NHTSA), investigates the separate and combined effects of alcohol and marihuana on driver performance and behavior. The research was motivated by the well-established link between alcohol and traffic accidents, contrasted with the unclear safety implications of marihuana, particularly given its increasing social acceptance. The study aimed to determine traffic safety implications and identify specific impairment mechanisms for both drugs alone and in combination. This report represents Phase II of a two-phase investigation, utilizing higher marihuana doses than Phase I to address previously inconclusive results. The researchers employed a fully interactive driving simulator that replicated a rural nighttime drive, allowing subjects full control over steering and speed. Twelve healthy subjects participated in a double-blind, full-placebo experimental design involving six sessions each. The design tested all combinations of two alcohol levels (0 and 0.10 percent Blood Alcohol Concentration) and three marihuana levels (0, 100, and 200 µg Δ9 THC/kg body weight). During each session, subjects completed a 10–12 mile drive lasting approximately 15 minutes, encountering various events such as curves, obstacles, and winding roads. Data collected included traffic safety measures (accidents, speeding tickets) and detailed driver behavior metrics (steering variability, speed control, reaction times). The results indicated that alcohol had a pervasive and significant impairing effect on driving performance. Specifically, alcohol increased simulator accidents, which was attributed to increased variability in steering and speed control, as well as increased reaction times. In contrast, marihuana effects were minimal; the primary observed effect was a statistically reliable but practically insignificant reduction in vehicle speed. Notably, a significant drug interaction effect was observed in the combined treatment group, resulting in increased simulator accidents. However, the data did not allow the researchers to identify the specific impairment mechanism responsible for this synergistic increase in accidents. No adverse subject reactions were observed at any dosage combination. The study concludes that alcohol at a 0.10 percent BAC significantly impairs driver control, leading to degraded traffic safety. While marihuana alone did not cause consistent impairment, the combination of alcohol and high-dose marihuana increased accident rates, highlighting a critical safety concern. The authors recommend further research to validate and explain the increased accident rate in combined drug conditions, suggesting the inclusion of blood plasma Δ9 THC concentrations in future studies. Additionally, they propose that road and vehicle designs should account for increased driver variability and reaction times, and that public education should emphasize the impairing effects of alcohol and its dangerous interaction with marihuana.

Key finding

Alcohol at 0.10 percent BAC significantly impaired driving by increasing steering and speed control variability, while the combination of alcohol and high-dose marihuana increased simulator accidents compared to alcohol alone.

Methodology

simulator

Sample size: 12

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• polydrug
• cannabis
• alcohol
• illicit drugs

Information type

What kind of knowledge this paper contributes, grouped by family — independent of topic (what it is about) and method (how it was studied).

• Empirical Findings: behavioral performance data
• Methodological Resource: validation psychometrics
• Theoretical Contribution: computational model