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 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.

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.