Field Sobriety Tests in Minnesota: Accuracy, Your Rights, and Defenses

You Can Refuse Field Sobriety Tests

This is the key practical point. Unlike the post-arrest chemical (breath) test — which is covered by Minnesota's implied-consent law and which you can be charged with a crime for refusing — roadside field sobriety tests are voluntary. You are legally allowed to decline them, and you cannot be charged with a separate crime or lose your license simply for saying no. The same goes for the roadside preliminary breath test (PBT) on the handheld device — refusing it is not a crime (this is different from the evidentiary breath test at the station).

Officers usually don't volunteer that these tests are optional — and in fact, most people cooperate without realizing they had a choice. A few important caveats:

• Refusing won't prevent an arrest if the officer already has probable cause from other observations (driving, odor, speech, eyes).
• A prosecutor may mention your refusal at trial as supposed "consciousness of guilt."
• Whether to perform or decline is a real-time judgment call — but knowing the tests are voluntary lets you make an informed one.

The Roadside PBT Cannot Be Used Against You at Trial

The roadside preliminary breath test (PBT) is not just voluntary to refuse — its result is also kept out of your criminal trial for most purposes. Under Minnesota's preliminary-screening-test statute, the PBT reading cannot be used as substantive evidence at trial to prove your alcohol concentration or that you were impaired. Its job is limited: it helps an officer decide whether there is probable cause to arrest and whether to require the evidentiary test at the station.

That limit is not absolute, and it is important to be precise about it. The statute allows the PBT result to be used for a short list of specific purposes — for example, to show that the officer was justified in requiring the post-arrest test, in a license-related civil proceeding, in an underage-drinking matter, and in a prosecution for test refusal. What it is not allowed to do is stand in as proof of your alcohol concentration in the DWI prosecution itself.

Two practical points follow. First, if the state tries to lean on the roadside number to argue you were over the limit, that is a challengeable use. Second, the PBT device is a screening tool — it is not the calibrated, evidentiary instrument used at the station, and it is not held to the same accuracy standards — so treating its reading as a reliable measurement is itself a weakness a defense can press.

The Three "Standardized" Tests

The National Highway Traffic Safety Administration (NHTSA) standardized three field sobriety tests, and these are the ones officers are trained to use:

• Horizontal Gaze Nystagmus (HGN) — following a pen or fingertip with the eyes while the officer watches for involuntary jerking;
• Walk-and-Turn (WAT) — taking heel-to-toe steps along a line, turning, and returning while following instructions; and
• One-Leg Stand (OLS) — standing on one foot and counting aloud.

Other roadside tasks officers sometimes use — the alphabet, counting backward, finger-count, finger-to-nose — are not NHTSA-validated and are considered less reliable.

How Accurate Are They, Really?

Not as accurate as people assume. In NHTSA's own validation research, the tests were only moderately accurate at identifying a target alcohol concentration: roughly 77% for HGN, 68% for walk-and-turn, and 65% for one-leg stand individually (with HGN and walk-and-turn combined around 80%). A few crucial points follow from this:

• These percentages mean a meaningful share of sober people still show "clues" and can be wrongly judged impaired.
• The tests were designed to estimate whether someone is over a set alcohol concentration — not to directly measure "impairment." Using performance to declare someone "impaired" can be challenged.
• The validation holds only when the tests are administered in the prescribed, standardized way, scored with the standardized clues, and interpreted by the standardized criteria. Deviations undermine the result.

The HGN Test — Especially Vulnerable

The HGN (eye) test is presented as scientific, but it rests on an officer's subjective observation of involuntary eye movement, and it has well-documented weaknesses:

• Nystagmus is a normal phenomenon in many people; alcohol may exaggerate it, but it isn't unique to alcohol.
• There are many types and causes of nystagmus — fatigue, certain medications, neurological and medical conditions, and even normal "end-point" nystagmus — that have nothing to do with alcohol.
• The NHTSA recognizes only three valid clues (lack of smooth pursuit; distinct nystagmus at maximum deviation; onset before 45 degrees), and using the test to estimate a specific alcohol concentration is a recognized misuse.
• Experts have criticized placing such a subtle medical judgment in the hands of minimally trained officers.

Proper administration matters greatly — the stimulus distance, speed, timing (holding at maximum deviation for several seconds), and sequence are all prescribed, and errors in any of them can render the result unreliable.

Factors That Skew Results — Even When Sober

Field sobriety test performance can be affected by many things unrelated to alcohol, including:

• Medical conditions, injuries, age, and weight;
• Nervousness and the stress of a police stop;
• Footwear and the road surface (gravel, slope, uneven pavement);
• Weather, lighting, and traffic conditions; and
• The officer's instructions and administration.

How Field Sobriety Tests Get Challenged

• Improper administration — deviations from the standardized NHTSA procedures;
• Scoring/interpretation errors — counting clues that don't qualify;
• HGN-specific problems — bad technique, or innocent causes of nystagmus;
• Innocent explanations — medical, physical, and environmental factors;
• Video contradictions — squad/body-cam footage that doesn't match the officer's narrative.

Because the legality of the stop and arrest also matters, FST challenges often work alongside challenges to the stop and the chemical test (see our pages on DWI arrests and breath testing).

What this means for you: Field sobriety tests can look damning in a police report, but they are subjective, only moderately accurate, and valid only when done by the book. A careful, technical review — ideally against any video — frequently reveals administration errors or innocent explanations that undercut the state's reliance on these tests.

Refusing the Evidentiary Test: Understanding the Trade-Offs

People often ask whether they should refuse the post-arrest evidentiary test (the breath test at the station, or a warrant-backed blood or urine test). This is a different question from the voluntary roadside tests above, and it carries real consequences that everyone should understand before a stop ever happens. This section explains those consequences as information — it is not a recommendation to refuse. Under Minnesota law, refusing the evidentiary test is itself a crime, and no responsible attorney can advise a person to commit that crime. What a lawyer can do is make sure you understand the landscape.

Here is what refusal actually involves:

• Refusal is a separate crime. Refusing the evidentiary breath test — or a blood or urine test backed by a search warrant — is charged as its own offense, and at the felony and gross-misdemeanor levels the refusal charge can carry penalties as serious as, or in some situations more serious than, the underlying DWI.
• It does not make the case disappear. The state can still prosecute the DWI using the officer's observations, driving conduct, and other evidence, and a prosecutor may argue that refusing shows "consciousness of guilt."
• Your license is still affected. Refusal triggers its own license-revocation consequences under the implied-consent laws, often longer than those that follow a test result.
• The warrant rules matter. A warrantless blood or urine test is treated differently from a breath test — there are situations where asking police to obtain a warrant is not itself the refusal crime. These distinctions are technical and fact-specific.

Because the consequences of testing and of refusing are both serious and depend heavily on the specific facts — your record, the type of test requested, whether a warrant exists — this is exactly the kind of decision to talk through with a lawyer. If you have the chance to consult counsel before deciding, use it; Minnesota gives you a limited right to contact an attorney before the evidentiary test, and that call is worth making.

Key Terms

• SFST: Standardized Field Sobriety Test — the three NHTSA-validated tests.
• HGN: Horizontal gaze nystagmus — the eye test, presented as scientific but subjective.
• Validation: Accuracy that holds only with standardized administration.
• Voluntary: FSTs (and the roadside PBT) can be refused without criminal or license penalty.
• Clues: The specific indicators officers count — only certain ones are recognized as valid.