THC and Hazard Perception: Why Feeling Worse Did Not Predict Test Performance
Medicinal cannabis users did not show a significant decline on a video hazard-perception test after oral THC, yet their own ratings did not accurately track their performance. This separates a measured driving-related skill from confidence about that skill.
The study’s actual comparison
Mieran and colleagues assessed 41 medicinal cannabis users in baseline and post-consumption conditions. Participants used their prescribed THC oil. The within-person design compared video-based hazard perception, selected risk-taking measures, and self-reported judgments.
These were standardized computer/video tasks, not actual collision outcomes or unrestricted public-road driving. A result on one task cannot establish that every driving demand was unaffected.
Different outcomes moved differently
Actual hazard-perception performance did not significantly decline. Participants nevertheless perceived their performance as worse. Their self-ratings did not significantly correlate with objective performance in either condition.
After consumption, participants selected slower speeds and longer following distances on the video measures, while gap acceptance was unchanged. Changes in general self-rated driving skills and safety were not significant after correction for multiple tests. Those findings should remain separate rather than being summarized as a uniform improvement or impairment.
Compensation is not proof of safety
Slower chosen speed or a longer chosen following distance can be consistent with compensation. Neither establishes that a person will compensate successfully for every impairment or unexpected traffic event. Likewise, a nonsignificant hazard-perception difference is not an equivalence demonstration or a universal waiting-period rule.
The selected medicinal-user population, prescribed products, within-subject design, and tested exposure conditions limit generalization to new users, different products, or different routes. The findings do not establish a blood or oral-fluid threshold for performance.
What matters in an individual review?
Identify the actual skill at issue, the person’s route and timing of use, baseline abilities, and observed behavior. A person’s statement that they feel impaired or unimpaired is evidence about their experience; it is not a calibrated measurement of their hazard-perception ability.
The paper is most useful for showing why subjective appraisal and objective task performance require separate analysis.
For related context, see our discussion of interpreting THC blood concentrations.
Primary research
Taren Mieran; Andrew Hill; Mark S. Horswill; Mathew J. Summers; Kayla B. Stefanidis. The effects of orally ingested Delta-9-Tetrahydrocannabinol on drivers’ hazard perception and risk-taking behaviours: A within-subjects study of medicinal cannabis users. Psychopharmacology (2025). DOI: 10.1007/s00213-025-06869-w.
