Cannabis Impaired Simulated Driving for Hours, but Users Felt Ready to Drive Too Soon | Research

The takeaway

Smoked cannabis impaired simulated driving for up to 3.5 hours in regular users, but 69% felt ready to drive at 1.5 hours despite continued impairment, suggesting a dangerous false sense of safety.

Cannabis users who drive, policymakers, traffic safety researchers

69% felt ready to drive despite ongoing impairment

What the researchers found

THC significantly impaired the Composite Drive Score at 30 minutes (d=0.59) and 1.5 hours (d=0.55), with borderline impairment at 3.5 hours (d=0.29) and no difference at 4.5 hours. Despite ongoing impairment, 69% of participants reported willingness to drive at 1.5 hours. Neither THC content (5.9% vs 13.4%) nor use intensity predicted driving impairment.

Why it matters

This is one of the largest randomized trials of cannabis and driving. The finding that experienced users cannot accurately judge their impairment has critical implications for road safety policies.

The numbers in context

191 cannabis users; mean age 29.9; mean 16.7 use days/month; Cohen d=0.59 at 30min; d=0.55 at 1.5hr; d=0.29 at 3.5hr; 69% willing to drive at 1.5hr; impairment resolved by 4.5hr

How the study worked

Double-blind, placebo-controlled parallel RCT at UC San Diego. 191 regular cannabis users randomized to placebo, 5.9%, or 13.4% THC smoked ad libitum. Driving simulator assessed at multiple timepoints through 4.5 hours.

What this study cannot tell us

Simulated driving, not real-world. Cannabis smoked ad libitum may not replicate all consumption patterns. Regular users only, so results may not apply to occasional users.

How to read the evidence

Well-designed double-blind RCT with 191 participants, multiple timepoints, and objective driving measures.

When this study was published

Published in 2022

The bigger picture

As cannabis legalization expands, determining safe wait times before driving is essential. This study suggests impairment persists longer than users perceive, with a dangerous gap between felt readiness and actual performance.

Questions still open

Should cannabis products carry minimum wait-time warnings before driving? Would different consumption methods (edibles, vaping) produce different impairment timeframes?

Common questions

How long does cannabis impair driving?

This RCT found significant impairment at 30 minutes and 1.5 hours after smoking, borderline impairment at 3.5 hours, and no impairment at 4.5 hours. The authors suggest most users should wait at least 4.5 hours before driving.

Does stronger cannabis impair driving more?

Surprisingly, no. When experienced users controlled their own intake, driving impairment was similar whether they smoked 5.9% or 13.4% THC cannabis, suggesting users titrate their dose regardless of potency.

Read the original research

Driving Performance and Cannabis Users' Perception of Safety: A Randomized Clinical Trial.

JAMA psychiatry, 79(3), 201-209

View on PubMed ↗Publisher’s paper ↗

Citation

Marcotte, Thomas D; Umlauf, Anya; Grelotti, David J; Sones, Emily G; Sobolesky, Philip M; Smith, Breland E; Hoffman, Melissa A; Hubbard, Jacqueline A; Severson, Joan; Huestis, Marilyn A; Grant, Igor; Fitzgerald, Robert L. (2022). Driving Performance and Cannabis Users' Perception of Safety: A Randomized Clinical Trial.. JAMA psychiatry, 79(3), 201-209. https://doi.org/10.1001/jamapsychiatry.2021.4037