While no clear linear relationship existed between blood THC levels and driving impairment, specified legal thresholds in blood (5 ng/mL) and saliva (25 ng/mL) did detect increased lane weaving after cannabis use.
Read this if you drive after using cannabis or are interested in how THC impairment is measured.
5 ng/mL bloodLegal THC threshold that detected significant driving impairment in simulator testing
What the researchers found
Researchers used a driving simulator to measure lane weaving (SDLP) after participants smoked cannabis alone or with alcohol. There was no clear linear dose-response relationship between blood THC concentration and impairment. However, when THC levels exceeded the legal thresholds (5 ng/mL in blood, 25 ng/mL in saliva), SDLP was significantly increased compared to placebo. The combination of cannabis and alcohol when above blood THC thresholds also showed increased impairment.
Why it matters
Cannabis-impaired driving is a major public safety concern, but unlike alcohol, there is no universally accepted impairment threshold for THC. This study provides evidence that existing legal cutoffs may have some validity for detecting impairment, even without a simple linear dose-response relationship.
The numbers in context
- Blood THC threshold tested: 5 ng/mL
- Urine THC-COOH threshold: 50 ng/mL
- Saliva THC threshold: 25 ng/mL
- No linear relationship between blood THC and SDLP (Spearman coefficients)
- Significant increase in SDLP above blood and saliva thresholds
How the study worked
Participants smoked cannabis alone or with alcohol in a controlled setting. THC/THC-COOH levels were measured in blood, urine, and saliva. Driving impairment was assessed via standard deviation of lateral position (SDLP) on a driving simulator. Results compared to placebo conditions.
Who was studied
Adult participants in a controlled cannabis and alcohol driving study
What this study cannot tell us
Driving simulator performance may not perfectly predict real-world driving impairment. Individual tolerance to THC varies greatly and was not fully accounted for. The study used smoked cannabis; other consumption methods may produce different pharmacokinetic profiles.
How to read the evidence
Controlled experimental study with objective outcome measures. Strong internal validity but limited ecological validity (simulator vs. real driving).
When this study was published
Published in 2023. Addresses an active area of cannabis policy research.
The bigger picture
As cannabis legalization expands, establishing defensible impairment standards is critical for traffic safety. This study suggests that per se limits in blood and saliva, while imperfect, do capture meaningful impairment — similar to how alcohol per se limits work despite individual variation.
Questions still open
- Would chronic cannabis users show the same SDLP increases at these thresholds?
- Could roadside saliva testing be more practical than blood testing for law enforcement?
- What is the optimal THC threshold that balances sensitivity and specificity for impairment detection?
Read the original research
The Utility of THC Cutoff Levels in Blood and Saliva for Detection of Impaired Driving.
Cannabis and cannabinoid research, 8(3), 408-413
Citation
Di Ciano, Patricia; Brands, Bruna; Fares, Andrew; Wright, Madison; Stoduto, Gina; Byrne, Patrick; McGrath, Michael; Hasan, Omer S M; Le Foll, Bernard; Wickens, Christine M. (2023). The Utility of THC Cutoff Levels in Blood and Saliva for Detection of Impaired Driving.. Cannabis and cannabinoid research, 8(3), 408-413. https://doi.org/10.1089/can.2022.0187