When police suspect you of driving while intoxicated, they typically ask you to perform roadside field sobriety tests (FSTs). These tests—the walk-and-turn, the one-leg stand, and the horizontal gaze nystagmus test—are presented as objective measures of impairment. In reality, they are highly subjective, often improperly administered, and frequently misinterpreted. Understanding their flaws is central to any effective DWI defense.
What Are Field Sobriety Tests?
There are three standardized tests. In the horizontal gaze nystagmus (HGN) test, the officer moves a stimulus across your field of vision and watches for involuntary jerking of the eyes. In the walk-and-turn test, you take a set number of heel-to-toe steps along a line, turn, and return while the officer watches for swaying, stepping off the line, using your arms for balance, or miscounting. In the one-leg stand test, you balance on one leg while counting aloud as the officer watches for swaying, hopping, or putting your foot down.
The Major Flaws in Field Sobriety Tests
The Tests Are Not Fully Validated
Although the National Highway Traffic Safety Administration (NHTSA) validated these tests through research, that validation has real limitations. It rested on relatively small samples, was tied to specific blood-alcohol levels rather than the full range of impairment, and did not adequately establish how often sober people fail. The result is a tool that is far less objective than it appears.
Many Sober People Fail
A substantial share of completely sober people fail field sobriety tests for reasons unrelated to alcohol. Physical conditions such as arthritis, back injuries, and inner-ear problems; neurological conditions; age and fitness level; awkward footwear; uneven or slick road surfaces; poor weather and lighting; and the simple nervousness of a police encounter can all produce “failing” performance in a person who has had nothing to drink.
The Tests Are Highly Subjective
Even with standardized scoring, administration and interpretation remain subjective. Whether you “stepped off the line,” how many steps you took, and whether your eyes showed genuine nystagmus are all matters of the officer’s judgment—judgment that is easily colored by an officer who already suspects impairment.
The Tests Are Frequently Misadministered
Officers often depart from NHTSA standards by failing to explain instructions properly, skipping the required demonstration, conducting tests on unsafe or uneven surfaces, ignoring poor lighting, and failing to account for physical conditions that affect performance. Compounding this, many officers receive minimal training, leading them to misread nystagmus, miscount steps, and misidentify ordinary nervousness as impairment.
Impairment Indicators Are Non-Specific
Many behaviors officers treat as signs of intoxication—swaying, poor balance, hesitation—are equally consistent with anxiety, poor coordination, medical conditions, fatigue, and age. A person with arthritis may sway; a person with an inner-ear condition may struggle to stand on one leg. Neither indicates intoxication.
Challenging Field Sobriety Tests
An effective challenge attacks both administration and interpretation. Where the surface, instructions, or conditions were improper, the reliability of the results can be challenged outright. Cross-examination should probe the officer’s qualifications—how many hours of FST training they received, when, and whether they have had any refresher—as well as exactly how they scored your performance: how many steps they counted, what they considered stepping off the line, and whether they noticed any physical conditions that could affect balance.
Expert testimony is especially powerful here. A biomechanist, neurologist, or forensic scientist can explain the limited scientific validity of FSTs, how often sober people fail them, the non-intoxication explanations for poor performance, and the specific ways the officer departed from standardized procedures. Where a medical condition affects your balance, coordination, or eye movement, addressing it directly explains poor performance without any inference of intoxication.
Field Sobriety Tests Are Not Required
A critical point many drivers do not know: you are generally not required to perform field sobriety tests, and declining them does not trigger the license suspension that can follow a refusal of chemical (breath or blood) testing. Because performing FSTs mainly creates subjective evidence the prosecution can use against you, many DWI defense attorneys advise clients that there is little to gain by attempting them.
Conclusion
Field sobriety tests are inherently unreliable—frequently misadministered, often misinterpreted, and commonly failed by sober people. If you performed FSTs during a DWI stop, an aggressive challenge to their reliability through cross-examination and expert testimony can be a powerful part of your defense. Contact The Ambeau Law Firm to discuss how your tests were administered and how to challenge them.