Field Sobriety Test Lawyer San Antonio: What They Actually Prove

A field sobriety test lawyer San Antonio drivers call after a roadside stop on Bandera Road or Broadway starts by asking what the officer actually did. Not whether the driver “passed” or “failed.” Those words do not appear in the manual. The three standardized tests come from the National Highway Traffic Safety Administration. Each one works by counting clues against a fixed threshold. NHTSA validated the tests at a specific accuracy rate, under specific conditions, with officers following a specific script. As a result, every step away from that script is a place to question the result in a Bexar County courtroom.

The Three Tests and How They Are Scored

The NHTSA standardized battery has three parts. First, the horizontal gaze nystagmus test checks for involuntary jerking of the eyes as they follow a stimulus. It has six possible clues, three per eye. The manual treats four or more as the sign of an alcohol concentration at or above 0.08. Second, the walk-and-turn test has eight clues, from starting too soon to stepping off the line. Two or more is the decision point. Third, the one-leg stand has four clues, including swaying, hopping, and putting the foot down. Again, two or more is the decision point. Therefore, an officer who observes fewer clues than the threshold has, by the manual’s own terms, not found evidence of impairment on that test.

Why a Field Sobriety Test Lawyer San Antonio Residents Hire Reads the Validation Study

The accuracy figures come from NHTSA’s own research. In the San Diego validation study, the horizontal gaze nystagmus test correctly identified drivers at or above 0.08 about 88 percent of the time. The walk-and-turn came in at 79 percent, and the one-leg stand at 83 percent. Those numbers describe the tests at their best, with trained officers on level ground and controlled conditions. In other words, the walk-and-turn misclassified roughly one in five. In a field setting on a sloped shoulder at night, with a nervous driver in boots, the real-world margin is wider. So the tests are evidence of impairment, not proof of it.

What Texas Courts Let an Officer Say About HGN

The Texas Court of Criminal Appeals addressed the eye test in Emerson v. State, 880 S.W.2d 759 (Tex. Crim. App. 1994). The court allowed HGN evidence when the officer completed a NHTSA-approved training course and followed the standardized procedure. However, it drew one firm line. An officer may testify that the driver showed signs of intoxication. That same officer may not testify that the eye test showed a particular alcohol concentration, because the court found the conversion formula carried too large a margin of error. Consequently, a San Antonio officer who tells a jury the driver was “at least a point one” based on the eyes alone has crossed that line.

Conditions a Field Sobriety Test Lawyer San Antonio Drivers Hire Checks

The manual instructs officers to screen for conditions that produce clues without alcohol. For instance, head injuries, inner ear problems, certain medications, and some neurological conditions all cause nystagmus. Likewise, age, weight, back or leg injuries, and footwear affect the balance tests. The manual also assumes a reasonably level, dry, well-lit surface. A test on the shoulder of Loop 1604 with traffic passing at highway speed does not meet that description. Dash and body camera footage from SAPD or Bexar County deputies often shows the ground, the lighting, and the instructions as given. For that reason, the footage is the single most useful piece of evidence in a field sobriety dispute.

Roadside Tests Are Not Covered by Implied Consent

Texas Transportation Code Section 724.011 deems a driver to have consented to a specimen of breath or blood after a DWI arrest. It says nothing about standing on one leg. Field sobriety tests are voluntary, and Texas attaches no license suspension to declining them. Still, the practical picture is more complicated. An officer who cannot gather clues from the tests may rely on other observations to justify an arrest. Prosecutors may also try to characterize a refusal for the jury. In short, declining the tests is a lawful choice with its own trade-offs, not a free pass. Arizona treats these roadside exercises much the same way, as our guide to field sobriety tests in Arizona explains.

The Bottom Line

Field sobriety tests measure clues, not intoxication. Officers count those clues against thresholds that NHTSA set, at accuracy rates that leave real room for error, under limits that Emerson v. State drew three decades ago. Meanwhile, the officer’s training, the surface, the lighting, the instructions, and the driver’s physical condition all bear on whether the count means anything. A field sobriety test lawyer San Antonio drivers retain is usually comparing the camera footage to the manual, one step at a time.

Keep reading: Field Sobriety Test Arizona: What They Actually Measure · DUI Traffic Stop Arizona: What Officers Are Allowed to Do

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This article is for general informational purposes only and does not constitute legal advice. How a court treats field sobriety test evidence depends on the officer’s training, the conditions of the test, and the rest of the record in the case.

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