By Graham Norris

When a Texas DWI case moves toward trial or active negotiation, two categories of evidence carry the most weight: the officer’s account of how the driver performed on field sobriety tests, and the BAC number from the Intoxilyzer 9000. Both look authoritative in the arrest report. Neither is as reliable as prosecutors want jurors to believe.

The three standardized field sobriety tests have documented accuracy limitations — limitations that widen when officers deviate from protocol. The station breath test depends on maintenance schedules, certification requirements, and a pre-test observation period that must be completed precisely or the reading loses its evidentiary footing. Attacking both categories of evidence often determines the outcome of a Fort Worth DWI case.

Graham Norris has successfully defended many clients against DWI charges in Tarrant County courts, securing dismissals, reduced charges, and favorable outcomes even in serious felony cases. Contact us now for a free case evaluation using our confidential online form, or call the number below.

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• Local Tarrant County Court Experience: We understand Fort Worth’s court system and have relationships with local prosecutors
• Proven Results: Over 400 case dismissals and 60+ grand jury “no-bills” across 3,600+ criminal cases
• Personal Attention: Your case isn’t just another file – Graham personally handles your defense
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The Three Standardized Field Sobriety Tests

The National Highway Traffic Safety Administration developed three standardized tests — Horizontal Gaze Nystagmus (HGN), Walk-and-Turn, and One-Leg Stand — to help officers establish probable cause for DWI arrest. When administered exactly as NHTSA protocol requires, NHTSA’s own research puts their standalone accuracy at 77%, 68%, and 65% respectively. Those figures were generated at a 0.10 BAC threshold — not the current Texas legal limit of 0.08 — meaning the tests are even less precise at the level that governs most prosecutions.

Protocol compliance is what gives these tests whatever reliability they have. The HGN requires the officer to move the stimulus at a specific pace and hold at maximum lateral deviation for a minimum of four seconds per eye. The Walk-and-Turn and One-Leg Stand require a flat, dry surface, proper verbal instructions, and a full demonstration before scoring begins. A deviation at any step cuts the scientific foundation out from under the result.

Conditions That Undermine FST Evidence

Physical and medical factors unrelated to alcohol produce the same clues officers score. Inner ear disorders affect balance-dependent tests. Certain medications cause nystagmus independent of alcohol. Prior injuries, age, weight, fatigue, and the stress of a roadside stop at night all affect performance in ways the scoring sheet never captures. Surface conditions and lighting matter too, though officers rarely document them — a defense attorney reviews dash cam footage to establish exactly where the test occurred and whether the conditions match the report.

Texas drivers may decline to perform field sobriety tests before arrest — they are voluntary. Refusing removes this evidence from the State’s case, though prosecutors will argue the refusal reflects consciousness of guilt.

The Intoxilyzer 9000 and Where It Can Fail

The Intoxilyzer 9000 is Texas’s primary evidential breath testing instrument, using infrared spectrometry to estimate BAC from a deep-lung breath sample. Unlike the portable PBT used at the roadside — inadmissible as substantive evidence of intoxication — the station result can go to a jury.

For that result to hold up, several conditions must be satisfied: the machine’s calibration records must show readings within acceptable tolerance; the DPS Technical Supervisor must have performed and documented required monthly maintenance; the administering officer must hold a current certification; and a 15-minute observation period must be completed and logged before the sample is taken, to rule out mouth alcohol and other contamination.

Gaps in calibration logs, missed maintenance intervals, lapsed certifications, or a shortened observation period can each support a motion to suppress the result. Pulling those records through open records requests is one of the first steps in building a defense.

The Rising BAC Problem

The Intoxilyzer 9000 measures BAC at the time of the test, not at the time of driving. Alcohol absorption takes 30 to 90 minutes after the last drink. A driver who registers 0.09 at the station an hour after a traffic stop may have had a BAC well under the legal limit when the car was actually moving.

Prosecutors bridge this gap through retrograde extrapolation — calculating backward from the test result to estimate BAC at the time of driving. The Texas Court of Criminal Appeals held in Mata v. State (2001) that this testimony is only admissible if the State’s expert accounts for the individual factors affecting that person’s specific absorption and elimination rate: weight, gender, food consumption, and the precise timing and amount of alcohol consumed. When the prosecution cannot supply those specifics — which is common — the extrapolation lacks the foundation required by Texas courts.

How This Evidence Gets Challenged

A defense attorney treats the field tests and the breath test as separate targets, attacking each on its own terms while looking for points where video contradicts the report. Dash cam and body cam footage are reviewed against the officer’s written account. Maintenance records are compared against the DPS documentation schedule. Medical history affecting balance or eye movement is gathered before trial.

The ALR hearing — which must be requested within 15 days of arrest — provides the earliest opportunity to cross-examine the arresting officer under oath. What surfaces there often shapes the entire criminal defense and can set the stage for DWI dismissal.

Frequently Asked Questions

Can I refuse field sobriety tests in Texas?

Yes. Field sobriety tests are voluntary before arrest, and you are not legally required to participate. Refusing eliminates the scored-clue evidence from the State’s case. Prosecutors may argue the refusal shows consciousness of guilt, but that argument is generally less damaging than a failed test on video.

Is the portable breath test (PBT) admissible in court?

No. The handheld PBT used at the roadside is not admissible as substantive evidence of intoxication in Texas. Only the station test on a certified Intoxilyzer 9000 can go to a jury.

What is the 15-minute observation period?

Before administering the Intoxilyzer 9000, the officer must continuously observe the subject for at least 15 minutes to ensure nothing enters the mouth that could contaminate the sample. If this period was shortened, interrupted, or not documented, the breath test result can be challenged.

Can a BAC over 0.08 still be beaten?

Yes. A number above 0.08 is not an automatic conviction. Calibration errors, maintenance gaps, observation period violations, rising BAC, and chain-of-custody issues all create paths to suppression or acquittal.

Do medical conditions affect field sobriety test results?

Absolutely. Inner ear disorders, knee or back injuries, neurological conditions, certain medications, and even contact lenses can produce the same “clues” officers score as signs of intoxication — without any alcohol involvement.

The Evidence Is a Starting Point, Not a Verdict

A breath test result and a failed field sobriety report are what an arrest looks like on paper. They are not what the case looks like once the records, footage, and science are pulled apart. Every piece of that evidence was created by a human being following a procedure, and every step in that procedure is a point where the defense can build reasonable doubt.

Graham Norris is a former Tarrant County prosecutor with more than 3,600 cases handled and over 400 dismissals. If you have been charged with DWI in Fort Worth or Tarrant County, call (817) 859-8985 for a free consultation.