How stoned drivers feel predicts driving performance beyond THC dose alone, but the specific predictors vary | Research

The takeaway

In a 52-person driving study, self-reported subjective effects of cannabis predicted driving performance beyond what THC dosing alone could explain, but the specific subjective feelings that mattered varied between studies.

Traffic safety researchers, policymakers working on cannabis-impaired driving laws, and cannabis users.

"Stoned" was the only perfectly consistent predictor of driving impairment

What the researchers found

Subjective perceptions of cannabis impairment significantly predicted driving performance measures (lane position, speed control) beyond the effect of THC dose. Feeling "stoned" was a perfectly consistent predictor across both this and a prior study. However, other subjective measures like "anxious," "good drug effect," and "restless" were inconsistent across studies, suggesting individual variability in the subjective-performance relationship.

Why it matters

If subjective feelings predict driving impairment better than blood THC levels, this has implications for impaired driving enforcement and personal decision-making about driving after cannabis use.

The numbers in context

52 subjects; placebo vs. 6.18% THC; "stoned" was a perfectly consistent predictor across both studies. "High" and "sedated" had one mismatch each. "Anxious," "good drug effect," and "restless" had three or more mismatches.

How the study worked

52 subjects completed a driving performance study with placebo vs. 6.18% THC cannabis. SAS GLM Select with stepwise selection analyzed relationships between subjective effects, dosing condition, and driving performance. Results compared to a prior 10-subject study.

What this study cannot tell us

Relatively small sample. Only one THC dose tested. Simulator driving may not reflect real-world conditions. Subjective measures are inherently variable. Comparison study had only 10 subjects.

How to read the evidence

Controlled driving study with replication attempt, but inconsistencies between studies limit generalizability.

When this study was published

Published in 2022.

The bigger picture

The inconsistency of subjective predictors between studies mirrors the challenge of cannabis-impaired driving enforcement: there is no simple equivalent of "I feel drunk" that reliably predicts impairment.

Questions still open

Could a validated subjective impairment scale be developed for cannabis? Would combining subjective reports with objective measures (like eye tracking) improve impairment detection? Does tolerance affect the subjective-performance relationship?

Common questions

Can drivers accurately tell if they are too impaired from cannabis to drive?

Subjective feelings predicted driving performance in this study, but the specific feelings that mattered varied. "Stoned" was the most reliable predictor, while feelings like "anxious" or "good drug effect" were inconsistent, suggesting self-assessment is imperfect.

Is THC blood level a good predictor of driving impairment?

THC dose alone predicted some driving impairment, but subjective effects explained additional variability in performance that blood levels could not, suggesting impairment is influenced by individual factors beyond just how much THC is in the blood.

Read the original research

Perceived effects of cannabis: Generalizability of changes in driving performance.

Traffic injury prevention, 23(sup1), S8-S13

View on PubMed ↗Publisher’s paper ↗

Citation

Burt, Thomas S; Brown, Timothy L; Schmitt, Rose; McGehee, Daniel; Milavetz, Gary; Gaffney, Gary; Berka, Chris. (2022). Perceived effects of cannabis: Generalizability of changes in driving performance.. Traffic injury prevention, 23(sup1), S8-S13. https://doi.org/10.1080/15389588.2022.2128787