Field sobriety tests feel like the moment your case is decided on the roadside. They aren't. The tests are voluntary in Texas, they were designed to estimate probability rather than prove impairment, and their weaknesses are some of the most documented territory in DWI defense.
The three standardized tests
The National Highway Traffic Safety Administration (NHTSA) recognizes three standardized tests. The horizontal gaze nystagmus (HGN) test tracks involuntary eye movement as you follow a stimulus. The walk-and-turn requires nine heel-to-toe steps along a line, a specific turn, and nine steps back. The one-leg stand requires balancing on one foot while counting aloud. Officers also use non-standardized tests — reciting the alphabet, finger-to-nose — which carry even less scientific weight.
Why the tests are often unreliable
NHTSA's own validation studies measured these tests under controlled conditions on dry, level surfaces with clear instructions — conditions rarely present on a highway shoulder at 1 a.m. with strobing patrol lights. Factors that degrade performance independent of alcohol include inner-ear and back injuries, knee and leg problems, age and weight, footwear, uneven or sloped pavement, weather, fatigue, and simple nervousness. HGN can also result from medical conditions and certain medications. Even correctly administered, the tests estimate a statistical likelihood of impairment — they do not measure your actual ability to drive safely.
How attorneys challenge them
The defense review starts with the officer's training and the video: were the standardized instructions read verbatim? Was the surface suitable? Were your medical issues asked about and documented? Deviations from NHTSA protocol undermine the tests' claimed reliability, and dash-cam and body-cam footage frequently contradicts the officer's report of "clues." The HGN test carries additional foundational requirements, and non-standardized tests get little weight.
Your roadside choices
Because the tests are voluntary, you can politely decline them — you'll likely still be arrested if the officer believes they have probable cause, but the state then has one less piece of evidence. That choice belongs to you in the moment; understanding the trade-offs ahead of time is the point of this page.
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