Neither blood tests nor field sobriety tests reliably detect cannabis-impaired driving
Both per se blood concentration limits and standard field sobriety tests have fundamental limitations for detecting cannabis-impaired driving, with poor correlation between THC levels and actual impairment and no validated functional test for cannabis.
Quick Facts
What This Study Found
Unlike alcohol, THC blood concentrations poorly correspond to amount consumed, crash risk, or degree of impairment. Standard field sobriety tests have not been developed or validated for cannabis, and cannabis impairment is difficult to distinguish from effects of aging or certain medications.
Key Numbers
Cannabis associated with approximately 2-fold increase in crash risk. Per se THC limits poorly predict impairment. No validated field sobriety test for cannabis. Cannabis impairment difficult to distinguish from aging or medication effects.
How They Did This
Perspective review examining the strengths and limitations of two cannabis-impaired driving detection approaches: per se blood/breath concentration tests and functional field sobriety tests, compared against their established use for alcohol detection.
Why This Research Matters
Cannabis use doubles automobile crash risk, yet neither of the two standard approaches for detecting impaired driving works reliably for cannabis. This leaves both cannabis users and law enforcement without clear guidance on driving fitness after use.
The Bigger Picture
The author argues that instead of trying to develop substance-specific detection methods for every possible intoxicant, general impairment detection strategies that work regardless of cause would be more practical and equitable.
What This Study Doesn't Tell Us
Perspective piece rather than systematic review. The 2-fold crash risk estimate varies across studies. Individual tolerance differences make any per se limit inherently imprecise. Technology may improve faster than this review anticipated.
Questions This Raises
- ?Could technology-based impairment assessments (reaction time apps, eye tracking) provide substance-agnostic detection?
- ?At what point after use is it actually safe to drive?
- ?Should legal frameworks shift from substance detection to impairment detection?
Trust & Context
- Key Stat:
- Cannabis doubles crash risk, but no reliable detection method exists for cannabis impairment
- Evidence Grade:
- Moderate: well-argued perspective synthesizing existing evidence, though not a systematic review.
- Study Age:
- Published in 2019.
- Original Title:
- Strengths and limitations of two cannabis-impaired driving detection methods: a review of the literature.
- Published In:
- The American journal of drug and alcohol abuse, 45(6), 610-622 (2019)
- Authors:
- Ginsburg, Brett C(3)
- Database ID:
- RTHC-02046
Evidence Hierarchy
Summarizes existing research on a topic.
What do these levels mean? →Frequently Asked Questions
Why can't police test for cannabis impairment like they do for alcohol?
Alcohol has a clear relationship between blood level and impairment. Cannabis does not. THC levels can be high without impairment or low while still impaired, depending on tolerance, timing, and individual metabolism.
What's the solution for cannabis-impaired driving detection?
The author suggests developing general impairment detection methods that work regardless of what substance caused the impairment, rather than trying to create specific tests for every possible intoxicant.
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Cite This Study
https://rethinkthc.com/research/RTHC-02046APA
Ginsburg, Brett C. (2019). Strengths and limitations of two cannabis-impaired driving detection methods: a review of the literature.. The American journal of drug and alcohol abuse, 45(6), 610-622. https://doi.org/10.1080/00952990.2019.1655568
MLA
Ginsburg, Brett C. "Strengths and limitations of two cannabis-impaired driving detection methods: a review of the literature.." The American journal of drug and alcohol abuse, 2019. https://doi.org/10.1080/00952990.2019.1655568
RethinkTHC
RethinkTHC Research Database. "Strengths and limitations of two cannabis-impaired driving d..." RTHC-02046. Retrieved from https://rethinkthc.com/research/ginsburg-2019-strengths-and-limitations-of
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.