In a driving simulator study with 118 participants using their own cannabis products, occasional users showed increased lane departures but daily users (including those using high-potency concentrates) showed minimal driving changes.
Traffic safety researchers, policy makers, law enforcement, daily cannabis users
Daily cannabis users showed minimal driving impairment even with high-potency concentrates
What the researchers found
Occasional flower users had a significant increase in lane departures after cannabis use (0.16/min at baseline to 0.47/min 30 minutes post-use). Daily flower and daily concentrate users showed little to no change in lane departures or speed. All cannabis-using groups maintained or slightly decreased speed post-use, while controls slightly increased speed. Overall driving changes were relatively small across all groups.
Why it matters
Most cannabis-driving research uses low-potency products that do not reflect what is actually sold in legal markets. This study used participants' own products (including high-potency concentrates) and found that tolerance plays a major role: daily users showed little impairment even with stronger products.
The numbers in context
118 participants; occasional flower users: lane departures increased from 0.16 to 0.47/min; daily flower and concentrate groups: minimal changes; controls slightly increased speed (53.9 to 54.6 mph); cannabis groups maintained speed; changes were relatively small across all groups
How the study worked
118 participants completed three 20-minute simulated drives: baseline, 30 minutes post-use, and 90 minutes post-use. 89 inhaled their own cannabis products (flower or concentrates) for up to 15 minutes, while 29 controls completed the protocol without using cannabis. Groups were divided by use frequency and product type.
What this study cannot tell us
Simulated driving may not capture all real-world driving demands, self-selected participants using their own products creates dosing variability, no THC blood levels reported in abstract, within-subjects design but unequal group sizes, controlled environment differs from real driving conditions
How to read the evidence
Well-designed within-subjects simulator study with naturalistic product use; moderate size but simulated rather than real driving
When this study was published
Published 2025
The bigger picture
The finding that daily users show minimal driving impairment even with high-potency products complicates impaired-driving policy. It suggests tolerance significantly affects driving risk, meaning a THC blood level that impairs an occasional user may have little effect on a daily user.
Questions still open
- Should impaired-driving thresholds account for tolerance? Are the small changes in occasional users enough to increase real-world crash risk? Would different driving scenarios (emergency braking, complex intersections) reveal larger differences?
Common questions
Does cannabis impair driving?
Does using stronger cannabis products mean worse driving?
Read the original research
Impact of naturalistic cannabis use on lateral control and speed: A driving simulator study.
Traffic injury prevention, 26(sup1), S86-S95
Citation
Brooks-Russell, Ashley; Bird, Sarah; Brown, Timothy; Limbacher, Sarah; Kosnett, Michael; Dooley, Greg; Wrobel, Julia. (2025). Impact of naturalistic cannabis use on lateral control and speed: A driving simulator study.. Traffic injury prevention, 26(sup1), S86-S95. https://doi.org/10.1080/15389588.2025.2508381
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