Visual Search and Urban City Driving under the Influence of Marijuana and Alcohol

Lamers, C. T. J.; Ramaekers, J. G. · 2000 · ROSA P / United States. National Highway Traffic Safety Administration

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Summary

This study investigated the separate and combined effects of low-dose marijuana (delta-9-tetrahydrocannabinol, THC) and alcohol on visual search behavior and driving proficiency in an urban environment. Motivated by epidemiological data indicating that marijuana users frequently drive under the influence of both substances, the research aimed to determine if the combination poses a greater traffic safety risk than either substance alone, despite previous findings suggesting minimal impairment from low doses of each individually. The experiment employed a balanced, four-way, crossover, observer- and subject-blind design involving sixteen recreational users of alcohol and marijuana. Participants underwent four treatment conditions on separate evenings: placebo for both substances, alcohol only (targeting a blood alcohol concentration [BAC] of approximately 0.05 g/dl), THC only (100 µg/kg), and the combination of both. The City Driving Test was conducted on a fixed 15 km route in Maastricht, Netherlands, lasting 45 minutes. Performance was assessed using two primary metrics: general driving proficiency rated retrospectively by a licensed instructor using the Royal Dutch Tourist Association’s Driving Proficiency Test, and visual search frequency at intersections measured via a head-mounted eye-tracking system. Results indicated that neither THC nor alcohol alone significantly impaired driving proficiency or reduced the frequency of visual searches for traffic at intersections compared to placebo. However, the combination of alcohol and THC significantly reduced visual search frequency by approximately 3% (p=0.041), with a more pronounced 7% drop observed in female subjects, though this gender interaction was only marginally significant. While the combined treatment did not affect the instructor’s rating of overall driving proficiency, it did impair peripheral attention. Additionally, alcohol alone significantly lowered performance on a route recognition memory test, whereas THC did not. Subjectively, participants reported feeling less alert and investing more effort when using THC alone, but these compensatory behaviors disappeared when alcohol was added, leading to a lack of perceived impairment despite the objective deficit in visual search. The study concludes that while low doses of THC and alcohol have minimal effects on driving proficiency and visual search when taken individually, their combination is potentially dangerous for traffic safety. The impairment manifests specifically as a reduction in peripheral visual search at intersections, suggesting that drivers under the combined influence fail to adequately monitor side traffic while maintaining central driving tasks. This finding highlights a specific hazard in urban driving environments where divided attention is critical, underscoring the risks of polydrug use even at low dosage levels.

Key finding

The combination of low-dose THC and alcohol significantly reduced visual search frequency for traffic at intersections by 3% compared to placebo, whereas neither substance alone caused such a reduction.

Methodology

on_road

Sample size: 16

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