Field Sobriety Tests for Marijuana and Drug DUIs

If an officer suspects you are driving high, the roadside ritual looks a lot like an alcohol stop: stand on one leg, follow a pen with your eyes, walk a straight line. But the science behind those tests does not translate cleanly to cannabis. Understanding what a field sobriety test for marijuana can and cannot prove helps you stay calm and protect your rights during a drugged-driving investigation.

What field sobriety tests actually are

The three Standardized Field Sobriety Tests (SFSTs) are the Horizontal Gaze Nystagmus (HGN) test, the Walk-and-Turn, and the One-Leg Stand. The National Highway Traffic Safety Administration (NHTSA) developed and validated this battery in the 1970s and 1980s to detect alcohol impairment, specifically to estimate whether a driver's blood alcohol concentration was above the legal limit. They were never validated to measure marijuana or other drug impairment.

This matters because cannabis affects the body differently than alcohol. HGN (the involuntary eye jerking the pen-tracking test looks for) is a recognized indicator of alcohol and certain depressants, but THC does not reliably produce it. The balance and divided-attention tasks can be failed by sober people who are nervous, tired, injured, overweight, elderly, or simply standing on gravel in the dark. An officer's conclusion that you "failed" is a subjective judgment, not a chemical measurement.

Why there is no roadside test for THC impairment

With alcohol, a portable breath test gives a number that correlates reasonably well with impairment. There is no equivalent for marijuana. Breath devices for THC remain experimental, and the bigger problem is biological: THC is fat-soluble and lingers in the blood and body long after the high wears off. A daily or medical user can register significant blood-THC levels days later while being completely sober, and an occasional user can be genuinely impaired with a low reading. Researchers, including NHTSA's own reports to Congress, have repeatedly found that blood-THC concentration poorly correlates with actual driving impairment. That single fact drives most of the legal fights in marijuana DUI cases.

Drug Recognition Experts (DREs)

When alcohol is ruled out but the officer still suspects impairment, departments often call a Drug Recognition Expert. A DRE performs a 12-step evaluation that includes checking pulse, blood pressure, pupil size under different lighting, muscle tone, the inside of the nose and mouth, and additional psychophysical tests, then forms an opinion about which category of drug is involved. DRE testimony is influential with juries but is not infallible science; defense attorneys regularly challenge it, and some courts limit how far a DRE can go in claiming a defendant was "under the influence" of a specific drug. A DRE opinion is still just an opinion that a chemical test is then supposed to confirm.

Per se limits versus impairment standards

States split into roughly three approaches for marijuana DUIs, so the rules genuinely vary by where you are:

• Per se THC limits. States like Colorado (a permissible-inference standard at 5 nanograms/mL), Washington, Nevada, Ohio, Pennsylvania, and Illinois set a specific blood-THC number. Above it, the state argues you are legally impaired regardless of how you actually drove.
• Zero-tolerance states. States such as Arizona, Georgia, Indiana, and Utah make it an offense to drive with any detectable THC (or its metabolites) in your system, which can capture people who used legally days earlier and are not impaired at all.
• Effect-based (impairment) states. Many states require the prosecution to prove your ability to drive was actually impaired, using the totality of the evidence rather than a magic number.

Because metabolites can persist for weeks, zero-tolerance and per se rules are the most criticized and the most often challenged on scientific grounds.

Are the roadside tests voluntary?

In most states the roadside SFSTs and any handheld preliminary screening are voluntary. Officers rarely tell you that. You can usually decline to perform balance tests, recite the alphabet, or count backward without an automatic license penalty, because those pre-arrest field tests are not the chemical test covered by implied-consent laws. The post-arrest evidentiary test (blood or urine for drugs) is different: under your state's implied consent law, refusing it after a lawful arrest typically triggers an administrative license suspension and can be mentioned at trial.

Performing physical sobriety tests is generally not protected by the Fifth Amendment because the U.S. Supreme Court treats your body and physical coordination as non-testimonial evidence (see Schmerber v. California and Pennsylvania v. Muniz). But the words you say can be testimonial, which is why answering "how much did you smoke?" is something you can decline using the right to remain silent.

Blood draws and your Fourth Amendment rights

A blood test is a search under the the Fourth Amendment. In Birchfield v. North Dakota, the Court held that police generally need a warrant to draw your blood and that states cannot make it a crime to refuse a warrantless blood test (unlike a breath test). Missouri v. McNeely held that the natural dissipation of intoxicants is not, by itself, an automatic exigent circumstances emergency that excuses a warrant, and Mitchell v. Wisconsin addressed warrantless draws from unconscious drivers. In practice, when you refuse, officers in many areas now phone a judge and get an electronic warrant within minutes.

What to do during a marijuana DUI stop

The stop itself must be supported by reasonable suspicion, and an arrest requires probable cause. Here is how to handle it calmly:

• Be polite, provide license, registration, and insurance, and avoid arguing on the roadside.
• You can decline roadside SFSTs and handheld pre-arrest screening in most states. A simple "I don't consent to field sobriety tests" is enough.
• Do not answer questions about what, when, or how much you used. Say you wish to remain silent and want a lawyer.
• Do not physically resist if you are arrested, even if you believe the stop was unlawful. Challenge it later.
• After a lawful arrest, understand that refusing the chemical (blood/urine) test carries implied-consent penalties; this is a judgment call best made knowing your state's rules.
• Write down everything you remember as soon as you can, including conditions, what the officer said, and whether you were read your Miranda rights.

This is general legal information, not legal advice. Marijuana DUI law varies significantly by state and turns on the specific facts of your stop. If you are charged, talk to a qualified DUI attorney in your state.