Field Sobriety Tests in California: What They Are and Why They're Unreliable

Reviewed by Ron Chini, Esq. · CA State Bar No. 263308 · DUI defense attorney in Irvine, CA since 2009.
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If you were pulled over for a suspected DUI in California, the officer almost certainly asked you to follow a pen with your eyes, walk a straight line heel-to-toe, or balance on one leg. These are field sobriety tests (FSTs) — and in California they are, for most drivers, voluntary. They are also far less scientific than they look, which is exactly why a careful defense can take them apart. This page explains what the roadside tests are, why they are unreliable, and how that becomes a defense. It is part of our broader DUI defense overview.

The three standardized field sobriety tests

The National Highway Traffic Safety Administration (NHTSA) recognizes only three "standardized" field sobriety tests. Officers across California are trained to administer and score these three in a specific way:

• Horizontal Gaze Nystagmus (HGN) — the officer moves a pen or finger side to side and watches your eyes for involuntary jerking. The theory is that alcohol exaggerates this jerking. It is the test jurors understand the least and the one most prone to being administered incorrectly.
• Walk-and-Turn (WAT) — you take nine heel-to-toe steps along a line, turn on one foot, and walk back. It is a "divided attention" test, meaning it is designed to make you balance and follow instructions at the same time.
• One-Leg Stand (OLS) — you stand on one foot, raise the other about six inches off the ground, and count aloud (often to 30) while keeping your arms at your sides.

The officer is not really scoring whether you "pass" — they are counting pre-defined "clues" (swaying, using your arms for balance, starting too soon, stepping off the line). Even a few clues can be written up as a fail.

Non-standardized tests carry even less weight

Officers sometimes add tests that NHTSA has never validated, such as the finger-to-nose test, the Romberg balance test (head tilted back, eyes closed, estimating 30 seconds), the finger-count test, or reciting part of the alphabet. Because these are not standardized, there is no agreed-upon scoring and no scientific accuracy data behind them at all. A skilled defense attorney can argue these prove little more than that you were nervous on the side of a freeway with flashing lights behind you.

Are field sobriety tests required in California?

For most drivers, no. Roadside field sobriety tests are generally voluntary, and there is typically no automatic license penalty for politely declining them (DRAFT — verify). This is very different from the chemical test (breath or blood) after a lawful DUI arrest, which California's implied-consent law generally does make mandatory, with real consequences for refusing (DRAFT — verify Veh. Code §23612).

Two important exceptions: drivers under 21 and drivers on DUI probation are generally required to submit to a roadside preliminary alcohol screening (PAS) breath test, and refusing it can trigger a license suspension (DRAFT — verify duration and authority). The PAS is a small handheld breath device — it is a chemical test, not a coordination test, so do not confuse it with the walk-and-turn or one-leg-stand. The DMV side of a DUI case moves on its own clock; see our guide to the DMV hearing.

Why the tests are unreliable

Even when scored by the book, these tests are imperfect. NHTSA's own validation research reports accuracy in only roughly the high-60% to high-80% range for the individual standardized tests, depending on the study and the test (DRAFT — verify exact figures and study citations). And those figures assume the officer administered everything exactly right — NHTSA itself warns that if the standardized procedures are not followed precisely, the results are compromised.

In the real world, plenty of sober, innocent things look exactly like "clues" of impairment:

• Nerves and fear — flashing lights, an armed officer, and the fear of arrest make almost anyone shaky and unsteady.
• Footwear — dress shoes, boots, sandals, or heels make a heel-to-toe walk genuinely difficult, sober or not.
• The surface and conditions — gravel, a sloped shoulder, uneven asphalt, rain, wind, and passing traffic all sabotage balance.
• Medical and physical conditions — inner-ear problems, back, knee, hip or leg injuries, neurological conditions, being overweight, older age, or fatigue can all cause "clues" that have nothing to do with alcohol.
• Eye and lighting issues — natural nystagmus, certain medications, and the strobing lights from the patrol car can mimic the HGN "clue" the officer is looking for.
• Officer subjectivity — there is no machine and no video score; the officer alone decides how much you swayed or whether you "stepped off the line."

How this becomes a defense

Because the tests are subjective and easy to administer wrong, they are a frequent point of attack. A defense built around the roadside tests typically looks at whether the officer gave the exact standardized instructions and demonstration; whether the location, footwear, and weather made a fair test impossible; whether a medical condition or injury explains the "clues"; and whether the officer's report matches what any in-car or body-camera video actually shows. Ron Chini has focused on California DUI defense since 2009 and reviews the stop, the instructions, and the scoring line by line — in English or Farsi — to find where the roadside evidence falls apart.

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Ron Chini, Esq.

DUI Defense Attorney · CA State Bar No. 263308

Ron has focused on DUI and criminal defense from his Irvine office since 2009. You work directly with him — not a paralegal or a call center.

Learn more about Ron Chini →