Can Cannabis Users Recognize Reduced Driving Ability? A Confidence Study
Cannabis users do not invariably believe they are driving normally after using THC. A controlled inhalation study found that driving confidence decreased alongside changes in simulated driving and vigilance. The result supports some awareness of effects, without establishing that self-assessment can certify an individual’s fitness to drive.
How confidence was compared with performance
Hartley and colleagues studied 30 healthy men aged 18 to 34: 15 chronic users and 15 occasional users. Participants received controlled cigarettes containing different THC doses or no THC. The cigarettes also contained tobacco.
The investigators measured self-rated driving confidence, sleepiness, psychomotor vigilance, simulator lane-position variability and blood THC. Their analysis included a pharmacokinetic–pharmacodynamic model linking exposure and responses over time.
The finding was reduced confidence, not universal unawareness
Confidence fell during the first two hours and remained below baseline at eight hours. Reduced confidence was associated with worse lane-control and vigilance measures. That pattern should not be rewritten as evidence that the participants uniformly failed to recognize their reduced abilities.
However, a relationship between average changes or model parameters is not a validated pass–fail rule for an individual. Confidence is subjective, and recognizing that performance feels worse does not establish the magnitude of impairment or when it has fully resolved.
A modeled compartment is not a measured brain concentration
The researchers used an effect-compartment model to account for a delay between measured exposure and observed effects. The model’s effect-site concentrations were inferred quantities. They were not directly sampled from brain tissue and should not be quoted as interchangeable with a measured blood THC threshold.
The all-male, young-adult sample, the specific cigarettes and the controlled setting limit wider application. The findings also do not establish a universal eight-hour waiting rule, since an observed group time course is not a personal clearance test.
Different tasks may relate differently to subjective experience. Our analysis of THC and hazard perception addresses that separate question. A careful opinion identifies the task, the self-rating and their actual relationship instead of assuming either perfect self-awareness or complete lack of insight.
Primary research
Can inhaled cannabis users accurately evaluate impaired driving ability? A randomized controlled trial. DOI: 10.3389/fpubh.2023.1234765.
