In a JAMA-published trial with actual on-road driving, THC-dominant cannabis significantly impaired lane-keeping for 40-100 minutes but not at 4 hours, while CBD-dominant cannabis did not impair driving at any time point.
Cannabis users who drive, traffic safety policymakers, and law enforcement.
THC impairment comparable to 0.05% BAC at 40-100 minutes
What the researchers found
THC-dominant and THC/CBD-equivalent cannabis significantly increased lane weaving (SDLP +2.33cm and +2.81cm respectively) at 40-100 minutes after vaporization. By 240-300 minutes, no significant impairment remained in any condition. CBD-dominant cannabis did not significantly affect driving at either time point. For reference, a 0.05% BAC increases SDLP by 2.4cm.
Why it matters
This is one of the only studies using actual on-road driving tests (not simulators) to measure cannabis impairment, published in JAMA, providing high-quality evidence for driving policy.
The numbers in context
26 participants (mean age 23.2, 16 women). 13.75mg THC and/or CBD doses. SDLP at 40-100 min: placebo 18.28cm, THC 20.59cm (+2.33), THC/CBD 21.09cm (+2.81), CBD 18.21cm (-0.07). At 240-300 min: no significant differences. 16 of 188 drives (8.5%) terminated for safety.
How the study worked
Double-blind, within-participants, randomized clinical trial at Maastricht University with 26 healthy occasional cannabis users. Four conditions: THC-dominant (13.75mg), CBD-dominant (13.75mg), THC/CBD-equivalent (13.75mg each), and placebo. On-road 100km driving tests at 40 and 240 minutes post-consumption.
What this study cannot tell us
Occasional users only (regular users may differ). Single dose tested. 8.5% of drives terminated for safety concerns. CBD-dominant effect size could not exclude clinically important impairment.
How to read the evidence
Rigorous double-blind RCT published in JAMA with real on-road driving assessment, the gold standard for driving impairment research.
When this study was published
2020 clinical trial.
The bigger picture
The finding that impairment resolves within 4 hours has implications for driving laws and workplace policies, though the doses tested may not represent all usage patterns.
Questions still open
- How long does impairment last at higher doses or in less experienced users? Should driving laws use time-based restrictions rather than THC blood levels?
Common questions
How long does cannabis impair driving?
Does CBD affect driving?
Read the original research
Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance: A Randomized Clinical Trial.
JAMA, 324(21), 2177-2186
Citation
Arkell, Thomas R; Vinckenbosch, Frederick; Kevin, Richard C; Theunissen, Eef L; McGregor, Iain S; Ramaekers, Johannes G. (2020). Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance: A Randomized Clinical Trial.. JAMA, 324(21), 2177-2186. https://doi.org/10.1001/jama.2020.21218