Field sobriety tests are frequently presented by prosecutors as objective proof of impairment. In reality, they are deeply susceptible to error — both on the part of the officer administering them and due to physical and medical factors entirely unrelated to alcohol or drug consumption. Understanding these factors is essential to building an effective DUI defense.

Factor One: Improper Administration by the Officer

The three standardized field sobriety tests — the Horizontal Gaze Nystagmus (HGN) test, the Walk-and-Turn test, and the One-Leg Stand test — are governed by specific protocols developed by the National Highway Traffic Safety Administration (NHTSA). These protocols dictate precisely how the tests must be explained, demonstrated, and administered. The tests are designated as “standardized” specifically because their validity as indicators of impairment depends on consistent administration. When an officer deviates from the standardized protocol, the reliability of the results is compromised.

Common administration errors include failing to demonstrate the test before asking the subject to perform it, giving incomplete or incorrect instructions, not allowing the subject to ask clarifying questions, administering the test on an uneven or sloped surface, and failing to observe the subject for the required period during the HGN test. If a subject is told incorrectly how to perform a test and then performs it in the way they were instructed, their performance reflects the officer’s error — not impairment. Defense counsel should obtain the officer’s NHTSA training records and scrutinize the dashcam or bodycam footage of the stop to identify any deviations from the standardized protocol.

Factor Two: Physical and Medical Conditions

Numerous physical and medical conditions can impair performance on field sobriety tests in ways that have nothing to do with alcohol or drug consumption. Balance and coordination — the primary attributes tested by the Walk-and-Turn and One-Leg Stand — are affected by a wide range of medical factors.

Inner ear disorders such as vertigo, labyrinthitis, or Meniere’s disease directly affect balance and can cause nystagmus — the eye movement that the HGN test purports to measure as a sign of alcohol impairment. Neurological conditions including multiple sclerosis, Parkinson’s disease, and peripheral neuropathy affect coordination and gait. Orthopedic injuries or conditions affecting the knees, ankles, hips, or lower back can make the One-Leg Stand or Walk-and-Turn test physically impossible to perform correctly regardless of sobriety. Eye conditions including certain prescription medications, eye surgeries, and conditions such as amblyopia or nystagmus can affect HGN results independent of alcohol.

Factor Three: Age, Weight, and Physical Fitness

NHTSA’s own research on the standardized field sobriety tests acknowledges that certain populations are not ideal subjects for these evaluations. Individuals over the age of 65, individuals who are significantly overweight, and individuals with certain physical limitations may have difficulty performing the balance-dependent tests even when completely sober. These factors should be identified and presented to the jury as alternative explanations for any observed performance difficulties.

Factor Four: Environmental Conditions

The conditions under which field sobriety tests are administered matter significantly. Tests conducted on uneven pavement, gravel, grass, or sloped surfaces introduce instability that is unrelated to the subject’s sobriety. Poor lighting conditions, the presence of passing traffic, and the psychological stress of a roadside encounter with law enforcement — with patrol car lights flashing nearby — can all affect performance. NHTSA guidelines require that tests be administered on a reasonably dry, hard, level, non-slippery surface; deviations from this requirement are legitimate grounds for challenge.

An experienced DUI defense attorney will review all available video, the officer’s training history, and the client’s medical background to identify every applicable factor affecting field sobriety test performance and present those factors effectively to undermine the government’s impairment evidence.