The HGN Trap: Why Officers Almost Always Perform It Incorrectly in Ohio OVI Cases

When someone is stopped for suspected OVI in Ohio, the Horizontal Gaze Nystagmus ("HGN") test is often the first “scientific” tool the officer relies on. Most drivers know it only as “the eye test,” and officers often describe it as the most reliable field sobriety test. In reality, HGN is the most technical, the most error-prone, and the least consistently administered test in the entire NHTSA system. The moment an officer deviates from the required procedure, the results lose reliability. Yet officers almost never perform it correctly. Understanding how HGN works, why it is so vulnerable to error, and how courts evaluate it is essential in evaluating any OVI charge.

What the HGN Test Is Actually Supposed to Measure

HGN looks for involuntary eye movements that may appear when someone is impaired. The NHTSA manuals describe specific clues across three categories: lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus prior to forty-five degrees. In theory, the test requires strict control over stimulus speed, distance, timing, angles, and medical qualification.

In practice, officers rarely follow these requirements. The test has more mandatory steps than any other SFST, and every step has specific language, timing, and technique. If the officer does not execute the test precisely as trained, the results are not consistent with NHTSA’s own research.

Strict Timing and Distance Requirements That Officers Ignore

NHTSA requires exact distances and durations for each pass of the stimulus. For example, the stimulus must be held twelve to fifteen inches from the subject’s face, slightly above eye level, and moved at a standardized speed. Each pass must take a minimum amount of time, and the officer must complete specific observation periods before counting clues.

Most officers do not use a stopwatch. Many rush through the passes, hold the stimulus too close or too far, or vary speed in ways that invalidate the test. Even small deviations affect the eye’s response and undermine the reliability of the observations. When examined closely, body-cam footage often shows errors in nearly every phase of the test.

Failure to Conduct Required Medical Qualification

Before administering HGN, the officer must determine whether the subject’s eyes can track properly at all. This includes checking for equal tracking, equal pupil size, and resting nystagmus. NHTSA requires these steps because certain medical conditions, injuries, medications, or neurological issues can create nystagmus even when someone is sober.

Officers frequently skip or rush these medical qualification steps. Some conduct them incorrectly or do not document them at all. When an officer cannot reliably determine whether a subject was medically qualified for testing, the validity of every later observation is compromised.

Improper Instructions and Poor Positioning

NHTSA requires officers to give specific instructions, hold the stimulus at the correct distance and angle, and position the subject to minimize distractions. Small errors in posture, stance, or background lighting can influence the test. Wind, passing traffic, flashing lights, or uneven footing can also interfere with tracking.

Many HGN errors stem from poor positioning. Officers often stand too close, move the stimulus inconsistently, or point the pen in a way that obscures part of the visual field. These issues are rarely documented, but they are clearly visible in video when reviewed in slow motion.

The Myth of Objectivity: HGN Is Highly Subjective in the Real World

Although officers often describe HGN as a scientific measurement, it is observational and subjective. Officers must interpret subtle eye movements in real time, in the dark, while holding a pen or fingertip steady and moving it at a consistent speed. Even trained instructors disagree about what constitutes a “distinct” or “early” clue.

Because HGN is subjective and heavily dependent on officer technique, courts carefully scrutinize the administration of the test. When the officer fails to follow training, the weight of the evidence can be significantly reduced or excluded entirely.

Why HGN Errors Matter in an OVI Case

HGN often forms the backbone of the officer’s impairment narrative. If the eye test is flawed, the remaining evidence becomes far weaker. Many OVI cases rely on the officer’s interpretation of field sobriety tests, and undermining the credibility of one component affects the overall evaluation of impairment.

Exposing errors in HGN administration can:

• weaken probable cause

• reduce the strength of the prosecution’s evidence

• undermine the reliability of the officer’s testimony

• support motions to suppress or limit field sobriety evidence

• improve negotiation leverage and trial strategy

A proper review of the officer’s technique, timing, and compliance with NHTSA standards often reveals issues that are not obvious at first glance.

Conclusion

The HGN test carries an appearance of scientific authority, but its reliability depends entirely on precise administration. Officers are required to follow strict procedures, yet the reality is that most do not. Understanding the technical requirements and identifying deviations is essential in evaluating whether the eye test provides meaningful evidence of impairment.

If you are facing an OVI charge and HGN was part of your stop, Harmony Law can review the evidence, identify weaknesses, and provide the experienced defense you need. Contact Harmony Law to discuss your case and understand your options.