Marijuana, Alcohol and Actual Driving Performance
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Summary
This study empirically determined the separate and combined effects of Delta-9-tetrahydrocannabinol (THC) and alcohol on actual driving performance. The research was motivated by conflicting evidence from previous epidemiological data, which suggested a synergistic interaction between the drugs, and experimental studies using simulators or closed courses, which generally found only additive effects. To address the lack of realism in prior experimental designs, this study measured performance in a natural setting on real roads with normal traffic. Eighteen licensed drivers (aged 20–28) who regularly used both substances participated in a balanced, six-way, observer- and subject-blind, cross-over design. Subjects were administered weight-calibrated doses of THC (100 µg/kg and 200 µg/kg) via smoking marijuana cigarettes, and alcohol sufficient to achieve a peak blood alcohol concentration (BAC) of approximately 0.07 g/dl, sustained at 0.04 g/dl during testing. Placebo conditions for both substances were also included. Thirty minutes after smoking, subjects performed two standardized driving tests: a Road Tracking Test, measuring the ability to maintain a constant speed of 100 km/h and steady lateral position, and a Car Following Test, measuring reaction times and headway variability while following a vehicle executing acceleration and deceleration maneuvers. The results demonstrated that both THC doses alone and alcohol alone significantly impaired driving performance. The impairment magnitude was minor for alcohol and THC 100 µg/kg, and moderate for THC 200 µg/kg. However, combining either THC dose with alcohol severely impaired performance in both tests. Specifically, the combination of alcohol and THC 100 µg/kg produced lateral position variability equivalent to a BAC of 0.09 g/dl, while the combination with THC 200 µg/kg was equivalent to a BAC of 0.14 g/dl. Reaction times and headway variability increased by approximately 36–37% under the combined influence of alcohol and THC 200 µg/kg compared to placebo. Subject and instructor ratings of driving quality corroborated these objective findings, with instructors noting dangerous aberrant behaviors primarily in combined drug conditions. The study concludes that while THC alone causes dose-related impairment, its combination with moderate alcohol levels results in severe degradation of driving ability comparable to high levels of alcohol intoxication. Although the study did not find unequivocal evidence of a classic pharmacological synergistic interaction, the exponential rise in lane deviation errors suggests that the combined use significantly increases crash risk. The findings imply that the practical consequences of driving under the influence of both substances are severe, regardless of whether the interaction is strictly synergistic or additive.
Key finding
The combination of THC (100 and 200 µg/kg) with alcohol sufficient for a blood alcohol concentration of approximately 0.04 g/dl severely impaired driving performance, producing effects equivalent to a blood alcohol concentration of 0.09 to 0.14 g/dl.
Methodology
on_road
Sample size: 18
Provenance
The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via bulk_ingest_rosap on 2026-05-23 (6 acquisition events logged).
| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
| discover | success | rosap | — | — | 2 | 2026-05-23 |
| archive | success | — | — | — | 1 | 2026-05-23 |
| extract | success | cached | — | — | 2 | 2026-06-10 |
| clean | success | — | — | — | 1 | 2026-06-01 |
| chunk | success | — | — | — | 1 | 2026-06-01 |
| embed | success | — | — | — | 1 | 2026-06-02 |
| enrich | success | — | — | — | 1 | 2026-05-23 |
| promote | success | — | — | — | 1 | 2026-05-23 |
| summarize | success | llm | qwen3.6-27b-prismaquant | summ-v5 | 3 | 2026-06-10 |
| tag | success | vector_similarity | — | — | 19 | 2026-06-11 |
| verify | success | — | — | — | 2 | 2026-06-10 |
Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.
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Information type
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- Empirical Findings: behavioral performance data
- Methodological Resource: validation psychometrics