Unlike alcohol, there is no roadside test that can reliably measure current THC impairment. Police typically use a combination of observations and tests rather than a single definitive measurement.
Common methods include:
- Observing driving behavior: Swerving, delayed reactions, failure to maintain lane, or other unusual driving can provide a reason for a stop.
- Physical observations: Officers may look for signs such as bloodshot eyes, the odor of cannabis, slowed responses, difficulty following instructions, or changes in speech or coordination. These signs are not specific to cannabis and can have other causes.
- Field Sobriety Tests (FSTs): These standardized tests were originally developed for alcohol but are also used in suspected drug impairment cases. They often include:
- Horizontal Gaze Nystagmus (eye movement test) – less useful for cannabis than alcohol because THC typically does not cause the same eye movements.
- Walk-and-Turn.
- One-Leg Stand.
- Drug Recognition Expert (DRE) evaluation: In some jurisdictions, an officer with specialized training may conduct a more extensive assessment, including checking vital signs, eye characteristics, muscle tone, and divided-attention tasks, to determine whether drug impairment is likely.
- Oral fluid (saliva) testing: Some states and countries use roadside saliva tests to detect the presence of THC. A positive result generally indicates recent use but does not prove that a person is currently impaired.
- Blood testing: If an officer has probable cause and state law allows it (or with a warrant or consent, depending on the jurisdiction), a blood sample may be collected later. Blood tests measure THC concentration, but THC levels correlate poorly with actual impairment because THC can remain detectable long after its impairing effects have worn off.
Why THC is harder to test than alcohol
Alcohol has a well-established relationship between blood alcohol concentration (BAC) and impairment. THC behaves differently:
- THC enters and leaves the bloodstream relatively quickly.
- People can have low THC levels while still impaired, or measurable THC levels long after they are no longer impaired.
- Frequent cannabis users may have detectable THC in their blood or body for days or even weeks without being impaired.
Because of this, officers generally rely on the totality of the evidence—driving behavior, observations, field tests, and, where applicable, toxicology results—rather than a single roadside device.
The specific procedures and legal standards vary by jurisdiction, so what officers can do and what evidence is required differs from state to state or country to country.
