THC impaired driving for about 4 hours but CBD did not, in on-road driving test
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- THC impairment comparable to 0.05% BAC at 40-100 minutes
- Evidence Grade:
- Rigorous double-blind RCT published in JAMA with real on-road driving assessment, the gold standard for driving impairment research.
- Study Age:
- 2020 clinical trial.
- Original Title:
- Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance: A Randomized Clinical Trial.
- Published In:
- JAMA, 324(21), 2177-2186 (2020)
- Authors:
- Arkell, Thomas R(10), Vinckenbosch, Frederick(2), Kevin, Richard C(16), Theunissen, Eef L, McGregor, Iain S, Ramaekers, Johannes G
- Database ID:
- RTHC-02393
Evidence Hierarchy
Participants are randomly assigned to treatment or placebo groups to test cause and effect.
What do these levels mean? →Frequently Asked Questions
How long does cannabis impair driving?
In this on-road driving study, THC-dominant cannabis impaired lane-keeping at 40-100 minutes after vaping but not at 240-300 minutes (4 hours), though this was at a moderate dose in occasional users.
Does CBD affect driving?
CBD-dominant cannabis (13.75mg) did not significantly impair driving at any time point in this study, though the researchers noted the results could not fully exclude small effects.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-02393APA
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
MLA
Arkell, Thomas R, et al. "Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance: A Randomized Clinical Trial.." JAMA, 2020. https://doi.org/10.1001/jama.2020.21218
RethinkTHC
RethinkTHC Research Database. "Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving..." RTHC-02393. Retrieved from https://rethinkthc.com/research/arkell-2020-effect-of-cannabidiol-and
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.