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Testing

Field Sobriety Tests: What the Three Standardised Tests Measure

Written from primary sources · Editor-reviewed · Law current as of 2026-09-20
By the duistatelaws.com editorial team · Published 2026-09-20 · Last reviewed 2026-09-20 · 3 min read
4 primary sources cited on this page. How we verify our legal content
Roadside testing is scored against criteria set before the stop.

Key takeaways

  • Only three tests are standardized by NHTSA: horizontal gaze nystagmus, walk-and-turn, and one-leg stand. Everything else a driver is asked to do on the roadside is unstandardised.
  • Each test is scored on a fixed list of clues, not on the officer's overall impression.
  • The validation studies measured whether the tests predict a BAC above a threshold — not whether a driver is impaired in any general sense.

What makes a test standardized?

A standardized test has a fixed administration procedure, a fixed set of scored clues, and published validation data. Three tests meet that description. An officer asking a driver to recite the alphabet backwards is not administering a standardized test, and the result is not scored against anything.

The three tests

Horizontal gaze nystagmus

The officer looks for involuntary jerking of the eye as it tracks a stimulus moved horizontally. Six clues are available, three per eye.

Walk-and-turn

A divided-attention test: the driver must hold a position while listening to instructions, then walk a straight line heel-to-toe, turn as instructed, and return. Eight clues.

One-leg stand

The driver stands on one foot and counts aloud. Four clues.

What the validation actually showed

The battery was validated in a San Diego study run with seven officers from that department’s alcohol enforcement unit, testing whether the three tests together predicted a BAC below 0.10 percent.

Measure Reported result
Arrest decisions correct at BAC 0.04–0.08 94%
All decisions correct 80%
Single test in isolation 79–88%
Three tests combined 91%
HGN correlation with BAC r = 0.65
All three combined r = 0.69

Two things in that table are worth reading twice. HGN alone is the most predictive single test, and the combination beats any test on its own — which is why the battery is administered as a battery and why skipping one of the three weakens the whole result.

Note what the question was: does the battery predict a blood alcohol concentration above a threshold? That is not the same question as whether someone was driving badly, and it is not the same question as whether the person is impaired by something other than alcohol.

Why the distinction matters

Because the tests are scored on clues, the record of which clues were observed is the evidence. A report that records a conclusion without the underlying clues has skipped the part that makes the test a test.

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