After consuming a typical retail cannabis edible (~7.3 mg THC), participants showed only a brief decrease in speed at 2 hours with no other driving impairment measures affected, despite feeling intoxicated for up to 7 hours.
Traffic safety researchers; policymakers setting cannabis-impaired driving standards.
Subjective intoxication lasted 7 hours but objective driving impairment was limited to 2-hour mark
What the researchers found
Cannabis edibles produced a decrease in mean speed at 2 hours post-consumption but not at 4 or 6 hours. No changes were found in weaving (SDLP), maximum speed, speed variability, or reaction time at any time point. Blood THC peaked at ~2.8 ng/mL at 2 hours. Driving impairment was not correlated with blood THC levels. Participants felt subjectively intoxicated for 7 hours and were less willing to drive for up to 6 hours.
Why it matters
This is the first study of cannabis edibles and driving. Current legal THC driving limits are based on smoked cannabis research. Edibles produce lower blood THC levels but longer subjective intoxication, creating a mismatch between how impaired users feel and what objective driving measures show.
The numbers in context
Mean THC consumed: 7.3 mg. Blood THC at 2 hours: ~2.8 ng/mL. Significant speed decrease at 2 hours only. No changes in SDLP, max speed, SD of speed, or reaction time at any time. Subjective intoxication lasted 7 hours. Reduced willingness/ability to drive for 6 hours.
How the study worked
Counterbalanced crossover study with cannabis edible and no-THC/CBD control sessions. Participants drove a driving simulator before and after consuming their preferred legally purchased edible (mean 7.3 mg THC). Blood THC and metabolites were measured. Standard and dual-task driving assessments at 2, 4, and 6 hours.
What this study cannot tell us
Driving simulator does not replicate all aspects of real-world driving. The mean dose (7.3 mg) was below the maximum single-package limit, so higher doses may produce more impairment. Individual tolerance varies widely. Small sample typical of driving simulation studies.
How to read the evidence
Moderate: controlled crossover design with ecologically valid dosing, but limited by simulator setting and relatively low dose.
When this study was published
2024 study.
The bigger picture
The disconnect between subjective intoxication (7 hours) and objective driving impairment (2 hours, speed only) raises complex policy questions. Current blood THC limits may not capture edible-related impairment accurately, and relying on driver self-assessment may overestimate actual impairment from edibles.
Questions still open
- Would higher-dose edibles (e.g., 25-50 mg) produce greater driving impairment? Should edible-specific blood THC thresholds be developed for driving laws? Why is driving impairment not correlated with blood THC after edibles?
Common questions
Are edibles safer for driving than smoking?
Why was driving impairment not linked to blood THC?
Read the original research
The effect of cannabis edibles on driving and blood THC.
Journal of cannabis research, 6(1), 26
Citation
Zhao, S; Brands, B; Kaduri, P; Wickens, C M; Hasan, O S M; Chen, S; Le Foll, B; Di Ciano, P. (2024). The effect of cannabis edibles on driving and blood THC.. Journal of cannabis research, 6(1), 26. https://doi.org/10.1186/s42238-024-00234-y