Analysis of over 2,100 drug evaluations found that standardized field sobriety tests identified impairment in users of cannabis, stimulants, depressants, and narcotics, though with different patterns for each drug class.
Read this if you want to understand how roadside sobriety tests detect cannabis impairment.
All drug categories showed significant impairment on standardized field sobriety tests
What the researchers found
All drug categories, including cannabis, were significantly associated with impaired performance on standardized field sobriety tests. On the One Leg Stand test, users of all drug classes were significantly more likely to sway while balancing and use their arms for balance compared to drug-free individuals.
Cannabis users showed distinct patterns of impairment across the three test components. CNS depressant users showed the most impairment on the Horizontal Gaze Nystagmus test (lack of smooth pursuit and nystagmus at maximum deviation). On the Walk and Turn test, depressant, stimulant, and narcotic users had more difficulty maintaining balance during instructions.
Interestingly, drug-impaired individuals were significantly less likely to hop during the One Leg Stand test, suggesting they compensated by keeping their foot down rather than attempting more complex balance maneuvers.
Why it matters
As cannabis-impaired driving becomes a growing concern with legalization, law enforcement needs validated tools for detecting impairment. This study provides evidence that standard field sobriety tests, originally developed for alcohol, can also detect cannabis-related impairment, though the patterns differ from alcohol impairment.
The numbers in context
2,142 DEC evaluations analyzed. Drug categories: CNS stimulants, CNS depressants, narcotic analgesics, cannabis, and drug-free. All categories showed significant impairment on at least one SFST component.
How the study worked
Researchers analyzed data from 2,142 completed Drug Evaluation and Classification (DEC) evaluations. These evaluations are conducted by specially trained Drug Recognition Experts on suspected drug-impaired drivers. Performance on the three SFST components (Horizontal Gaze Nystagmus, One Leg Stand, Walk and Turn) was compared across users of CNS stimulants, CNS depressants, narcotic analgesics, cannabis, and drug-free cases using multinomial logistic regression.
What this study cannot tell us
The data came from DEC evaluations of individuals already suspected of impairment, introducing selection bias. The study could not assess sensitivity (how many impaired drivers pass the tests) or specificity in naturalistic settings. Cannabis was often used alongside other substances, making it difficult to isolate its specific effects. The study did not correlate SFST performance with THC blood levels.
How to read the evidence
This is a large cross-sectional analysis of field data. While the sample size is substantial, the data comes from individuals already suspected of impairment, limiting generalizability.
When this study was published
Published in 2014. The challenge of detecting cannabis-impaired driving remains an active area of research and policy discussion.
The bigger picture
Unlike alcohol, there is no simple blood or breath test that reliably correlates with cannabis impairment. Field sobriety tests offer a behavioral assessment of impairment regardless of the substance involved. This validation data supports their continued use as screening tools, though the specific impairment patterns vary by drug class.
Questions still open
- How sensitive are field sobriety tests for detecting mild cannabis impairment? Should the scoring criteria be modified for different drug classes? Can additional tests improve detection of cannabis-specific impairment?
Common questions
Can field sobriety tests specifically identify cannabis versus other drugs?
Are field sobriety tests reliable for cannabis?
Read the original research
An examination of the validity of the standardized field sobriety test in detecting drug impairment using data from the Drug Evaluation and Classification program.
Traffic injury prevention, 15(2), 125-31
Citation
Porath-Waller, Amy J; Beirness, Douglas J. (2014). An examination of the validity of the standardized field sobriety test in detecting drug impairment using data from the Drug Evaluation and Classification program.. Traffic injury prevention, 15(2), 125-31. https://doi.org/10.1080/15389588.2013.800638