Are Field Sobriety Tests Reliable?
When a police officer suspects a driver of impairment, one of the first tools they reach for is the field sobriety test (FST). These roadside assessments are designed to measure coordination, balance, and cognitive function — all of which can be affected by alcohol or drugs. However, field sobriety tests are far from a perfect science, and their results are frequently challenged in court.
If you've been charged with a DWI or DUI based in part on a field sobriety test, understanding the legal and scientific weaknesses of these tests could be an important part of your defense.
The Three Standardized Field Sobriety Tests
The National Highway Traffic Safety Administration (NHTSA) has standardized three field sobriety tests that law enforcement officers are trained to administer:
1. Horizontal Gaze Nystagmus (HGN)
The officer watches your eyes as they follow a moving object (like a pen or flashlight). They're looking for involuntary jerking of the eye (nystagmus), which can indicate alcohol impairment. However, nystagmus also occurs naturally and can be caused by neurological conditions, certain medications, and fatigue.
2. Walk-and-Turn (WAT)
You're instructed to walk heel-to-toe in a straight line, turn, and return. Officers look for signs like losing balance, stepping off the line, or using arms to steady yourself. Poor performance can result from physical conditions, uneven road surfaces, or simply nervousness.
3. One-Leg Stand (OLS)
You must stand on one leg for approximately 30 seconds while counting aloud. Balance issues, medical conditions, age, and footwear can all legitimately affect performance on this test.
Common Grounds for Challenging FST Results
Experienced DWI defense attorneys examine many factors when challenging field sobriety test evidence:
Improper Administration
NHTSA has strict protocols for how each test must be administered. If the officer didn't follow the standardized instructions — including how they explained the test, the surface used, or lighting conditions — the results may be inadmissible or significantly weakened in court.
Environmental Conditions
- Uneven pavement or gravel affecting balance tests
- Poor lighting that makes eye tracking difficult
- Extreme weather (cold, rain, wind) interfering with performance
- High-traffic roadside settings that cause distracting noise and anxiety
Physical and Medical Conditions
Many legitimate medical conditions can mimic the signs of impairment that FSTs measure:
- Inner ear disorders affecting balance
- Neurological conditions causing nystagmus
- Musculoskeletal injuries limiting mobility
- Obesity or age-related physical limitations
- Anxiety disorders affecting performance under pressure
Lack of Officer Training or Certification
Officers who administer FSTs are required to undergo NHTSA-approved training. Your attorney can request the officer's training records and certification history. If the officer was not properly certified or their training was outdated, this creates a significant challenge to the validity of their observations.
Subjectivity of the Tests
Unlike a breathalyzer reading, which produces a numerical result, FST evaluations are largely based on the officer's subjective judgment. Defense attorneys can argue — often effectively — that the officer's observations are open to interpretation and are not sufficient, on their own, to establish impairment beyond a reasonable doubt.
Non-Standardized Tests Can Be Challenged Even More Easily
Beyond the three NHTSA-standardized tests, officers sometimes ask drivers to perform other tasks — reciting the alphabet backward, touching their finger to their nose, or counting. These non-standardized tests have even less scientific backing and are highly susceptible to legal challenge.
What to Do If You Were Given a Field Sobriety Test
- Write down everything you remember as soon as possible — road conditions, lighting, what the officer said, any physical conditions you had that night.
- Request dashcam or bodycam footage through your attorney. Video evidence often contradicts an officer's written account.
- Disclose all medical conditions to your attorney, even if they seem unrelated — your attorney can determine what's legally relevant.
- Don't assume the tests are conclusive. They are one piece of evidence, not a verdict.
The Bottom Line
Field sobriety tests are tools, not proof. They are influenced by numerous factors outside of alcohol impairment, and their administration is subject to strict legal requirements. A skilled DWI defense attorney will scrutinize every aspect of how your FSTs were conducted — and may be able to get the results reduced in evidentiary weight or excluded entirely.