Field Sobriety Tests Measure Performance. Not Intoxication.

Standardized Field Sobriety Tests are one of the most widely used and misunderstood tools in DWI investigations.

These “tests” don’t measure alcohol concentration, drug concentration, or intoxication. Instead, they measure how a person performs during a series of roadside exercises and how an officer interprets that performance. This is an important distinction we magnify in every DWI case.

At Denuna & Westbrook, PLLC, we don’t accept an officer’s subjective opinion and conclusions at face value. We examine how the standardized Field Sobriety Tests were administered, the conditions under which they were performed, and whether the officer’s interpretation is actually supported by the evidence.

How Standardized Field Sobriety Tests Were Developed and Why That History Matters

Back in 1975, the National Highway Traffic Safety Administration (NHTSA) contracted with the Southern California Research Institute to identify reliable roadside sobriety tests. The result was a series of studies that became the foundation for what are now known as the Standardized Field Sobriety Tests, or SFSTs.

Prosecutors rarely tell juries what those studies actually showed, but we do.

The original 1977 study was conducted entirely in an indoor lab. Nearly two-thirds of the subjects were male. No field testing was conducted. Nobody was outside, on the side of a road. The studies were never submitted for peer review and were never published in any scientific journal. And the false arrest error rate, meaning the percentage of arrest decisions based on these tests alone that were simply wrong, was 47%. Nearly half.

The 1981 study reduced that error rate to 32%, but NHTSA’s lead researcher later admitted, under oath, that this result was unacceptably high by scientific standards. Her explanation was officer inexperience. She suggested they use inexperienced officers in the next study.

The 1983 field validation, originally planned as an 18-month nationwide study, became a three-month evaluation with four police agencies. Officers received one day of training. Data was collected on the honor system: officers self-reported their results with no independent verification that they had administered or scored the tests correctly.

None of these studies were peer-reviewed, and none were published in a scientific journal. This is the research foundation that prosecutors present to juries as reliable science. Clearly, it isn’t.

“Standardized” Is the Key Word and the Key Weakness

The three tests are called Standardized Field Sobriety Tests for a reason. NHTSA created uniform procedures governing exactly how every officer in Texas, and every officer across the country, must explain, demonstrate, administer, and score each test. The instructions are the same. The demonstrations are the same. The scoring criteria are the same. That uniformity is what is supposed to make the tests reliable.

It also means any deviation from those procedures is a problem the defense can use.

When an officer doesn’t follow the standardized procedures, the test is no longer standardized. And if the test isn’t standardized, the accuracy figures prosecutors cite in court, which were generated under standardized conditions, no longer apply. An improperly administered Standardized Field Sobriety Test isn’t a variation of the standardized version. It’s a different test entirely, one that has never been validated and carries no established accuracy rate.

That’s not a technicality. It’s a direct challenge to the tests and their alleged reliability.

The Legal Standard for Intoxication and the Limits of SFSTs

Under Texas Penal Code Section 49.04, the State must prove that a person operated a motor vehicle in a public place while intoxicated. Texas law defines intoxication three ways:

• Loss of normal mental faculties caused by alcohol, drugs, or another intoxicating substance
• Loss of normal physical faculties caused by alcohol, drugs, or another intoxicating substance
• A blood alcohol concentration of 0.08 or greater

Standardized Field Sobriety Tests fail to directly measure any of these required elements. They don’t measure alcohol or drug concentration. They don’t determine whether a person has lost the normal use of their mental or physical faculties. What they produce is an officer’s interpretation of performance on a series of roadside exercises, and the reliability of that interpretation depends on the officer’s training, the instructions given, the conditions present, and the person standing on the side of the road.

Understanding the Three Tests and Their Limitations

Most DWI investigations involve three specific exercises developed by NHTSA and standardized for use by law enforcement nationwide. Understanding what each test actually measures, and where each one falls short, is essential to evaluating the evidence in any DWI case.

The Horizontal Gaze Nystagmus Test

HGN is presented as the strongest of the three Standardized Field Sobriety Tests. Unlike the Walk-and-Turn and One-Leg Stand exercises, HGN doesn’t involve balance or coordination. The officer observes involuntary jerking of the eye as a stimulus moves back and forth. Because it doesn't require physical performance, officers and prosecutors treat it as more objective.

During the HGN test, the officer looks for six possible clues, three per eye:

• Lack of smooth pursuit as the eye follows the stimulus
• Distinct and sustained nystagmus at maximum deviation
• Onset of nystagmus prior to 45 degrees

If the officer observes four or more clues, the test points toward a DWI arrest. Four out of six is all it takes. But these clues don’t necessarily prove intoxication. Nystagmus has more than twenty documented causes that have nothing to do with alcohol. NHTSA’s own training materials identify neurological conditions, inner-ear disorders, certain medications, head injuries, and eye conditions as potential causes. A person taking prescribed anti-seizure medication could show four out of six HGN clues without a drop of alcohol in their system. Officers aren’t trained to distinguish between pathological nystagmus and alcohol-induced nystagmus during a roadside test. The clues may be real, but what caused them is another question.

The testing environment creates its own problems. A roadside DWI investigation is filled with moving and flashing visual stimuli, which can trigger involuntary eye jerking known as optokinetic nystagmus, a condition that mimics alcohol-related nystagmus. Officers administer the HGN test while standing next to their patrol vehicle with emergency lights flashing. If the subject is facing oncoming traffic, passing headlights create the same issue. These conditions can produce optokinetic nystagmus and may have nothing to do with alcohol. Yet the clues they generate look identical to what officers are trained to associate with intoxication.

The research behind the threshold itself raises further questions. One laboratory HGN study found that when the test was administered correctly, the false positive rate was 67%. When the stimulus was held too high, that rate climbed to 91%. The same study found that 68% of people with a BAC below 0.05 still showed four or more clues, the exact threshold officers use to recommend arrest.

Walk-and-Turn

The Walk-and-Turn is a divided attention test requiring a person to listen to instructions, remember a sequence of steps, maintain balance, and perform physical movements simultaneously. Prosecutors argue difficulty with divided attention signals intoxication.

The officer looks for eight possible clues:

• Cannot maintain balance during instructions
• Starts walking too soon
• Stops while walking
• Misses heel-to-toe contact
• Steps off the line
• Uses arms for balance
• Makes an improper turn
• Takes the wrong number of steps

Showing just two of these eight clues is the threshold officers use to conclude a BAC at or above 0.08. Two out of eight. What makes that threshold alarming is what NHTSA’s own research revealed. In their studies, at least one out of every three people who had consumed no alcohol whatsoever was still identified as intoxicated after performing this test. The test that prosecutors present as reliable failed sober people at a significant rate in the very studies used to validate it.

The conditions under which the test is administered make it even less reliable. NHTSA’s training manual specifies that Walk-and-Turn should be conducted on a reasonably dry, hard, level, non-slippery surface. But in practice, tests are frequently performed on gravel shoulders, grassy medians, and sloped pavements. Fatigue, stress, anxiety, medical conditions, injuries, age, footwear, uneven pavement, weather, and confusion about the instructions all affect performance, independent of intoxication.

One-Leg Stand

The One-Leg Stand requires a person to stand with one foot approximately six inches off the ground and count aloud until told to put the foot down. Like the Walk-and-Turn, it is a divided attention test designed to assess balance and the ability to follow multi-step instructions simultaneously.

The officer looks for four possible clues:

• Swaying while balancing
• Using arms for balance
• Hopping to maintain balance
• Putting the foot down before instructed

Two of those four clues is the threshold for concluding intoxication. But look closely at what those clues actually describe. The scoring criteria doesn’t require the officer to specify how much the person swayed, how far their arms moved, how many times they hopped, or exactly when they put their foot down. An officer can check “swaying” based on the slightest perceptible movement. An officer can check “used arms for balance” whether the arms moved six inches or six feet. The clues are recorded as present or absent, and the officer decides what counts.

The validation research makes that subjectivity even more troubling. NHTSA’s own studies found that two or more clues occurred in 41% of people with a BAC below 0.08. In a separate study, the One-Leg Stand produced a 37% false positive rate, meaning roughly one in three completely sober subjects was found to be intoxicated.

NHTSA’s training manual also acknowledges that people over 65, those who are 50 or more pounds overweight, and those with leg, back, or middle ear problems will have difficulty with this test even when completely sober. Officers are supposed to account for these factors before administering the test, but in reality, that rarely happens. A person standing on the side of a roadway at night is not operating under the controlled conditions the test was designed for, and the research shows it.

What Prosecutors Don't Tell Juries About the Accuracy Figures

When testifying, experienced officers may cite accuracy figures for each Standardized Field Sobriety Test:

• HGN: 88%
• Walk-and-Turn: 79%
• One-Leg Stand: 83%

Those figures come from the 1998 San Diego field study. What jurors rarely hear is that these figures reflect the combined battery administered by specially trained officers under controlled field conditions, not the accuracy of the individual tests in isolation. The original 1977 laboratory study, which evaluated the tests individually, produced a 47% error rate. The studies were also originally validated at a BAC of 0.10, not the current legal limit of 0.08.

The Officer Is Central to Each Field Sobriety Test

Law enforcement plays an active role throughout the SFST process. The officer gives the instructions, demonstrates the exercises, observes your performance, and decides whether specific behaviors constitute clues of intoxication.

This is not an objective process.

Unlike a breath test or blood test, it doesn’t produce a numerical result. The officer must interpret what they observed and decide whether it’s consistent with intoxication. The quality of that interpretation depends directly on the quality of the instructions, the demonstration, the observation, and the officer’s training. If instructions were unclear, demonstrations incomplete, or observations exaggerated, the reliability of everything that follows is compromised.

This is why body-camera footage, dash-camera recordings, and officer training records matter as much as the test results themselves.

Alcohol Tolerance and Absorption: The State’s Shifting Arguments

Field sobriety tests create a problem for the State that rarely gets examined openly. When someone’s performance is poor, the prosecutor argues the tests prove intoxication. When someone's performance is good, the prosecution may pivot entirely, arguing the driver performed well because of high alcohol tolerance, meaning they appear coordinated despite being intoxicated.

This creates a situation where good performance and poor performance can both be offered as evidence of the same conclusion. An experienced defense attorney who understands how prosecutors use these tests can expose that contradiction directly to a jury.

The absorption argument runs in a similar direction. Alcohol isn’t instantly distributed throughout the body after it’s consumed. A person who recently drank alcohol may still be in the absorption phase when field sobriety tests are administered, meaning their BAC is still rising. Blood or breath test results obtained 30 to 90 minutes after the stop may show a higher BAC than what was actually present while the person was driving. Prosecutors tend to ignore this issue. They present the test result as though it reflects the defendant’s condition at the moment of driving, despite the science.

The tolerance and absorption arguments reveal the same underlying problem. The State presents these tests as precise and objective but applies the science selectively, using it when it helps and ignoring it when it doesn’t.

How Denuna & Westbrook Challenges Standardized Field Sobriety Tests

Chris Denuna is a Forensic Lawyer-Scientist, a designation that reflects advanced training in the scientific and forensic issues that arise in DWI prosecutions, including Standardized Field Sobriety Test methodology, NHTSA procedures, and the research underlying these tests.

That training means Denuna & Westbrook approaches field sobriety evidence the way a scientist would: examining whether the officer followed the procedures they were trained to follow, whether the testing environment was appropriate, whether alternative explanations exist for what was observed, and whether the conclusions are actually supported by the evidence.

Nick Westbrook and Chris Denuna have successfully challenged Standardized Field Sobriety Test evidence at suppression hearings, at trial, and in negotiations. Dismissals, favorable resolutions, and Not Guilty verdicts have resulted from cases where field sobriety evidence was the prosecution’s primary tool. (→ Results)

Frequently Asked Questions — Standardized Field Sobriety Tests in Texas

Do I have to perform Standardized Field Sobriety Tests in Texas?

No. Standardized Field Sobriety Tests are voluntary in Texas. That said, refusing comes with its own consequences. Officers will note the refusal in their report and may use it as justification for an arrest, particularly if they already observed an odor of alcohol, bloodshot eyes, slurred speech, or unsteady movement. A refusal doesn’t prevent an arrest; it may simply shift what evidence the officer relies on to justify one.

Can a sober person fail Standardized Field Sobriety Tests?

Yes. Stress, fatigue, medical conditions, physical limitations, poor instructions, and environmental conditions all affect performance independent of intoxication.

Are Standardized Field Sobriety Tests actually scientific?

Not based on the studies and research. They are exercises that rely heavily on officer observation and interpretation. The foundational studies were never peer-reviewed, never published in scientific journals, and produced unacceptable error rates.

What if the officer says I showed multiple clues?

The existence of alleged clues doesn’t establish intoxication. The reliability of those observations depends on how the tests were administered, the conditions present, and whether alternative explanations exist for what the officer observed.

What happens if the officer didn’t follow the standardized procedures for the field sobriety tests?

When officers deviate from standardized procedures, the test is no longer standardized. A test administered differently carries no validated accuracy rate.

Can challenging Standardized Field Sobriety Tests help my case?

Because these tests drive the arrest decision and anchor the State’s theory of intoxication, weaknesses in the testing process can change how the entire case is evaluated. These are among the most effective areas of a DWI defense.

Contact a Houston Standardized Field Sobriety Test Defense Lawyer Today

A poor performance on a roadside exercise is not proof of intoxication. Understanding the history, the limitations, and the proper procedures behind these tests is the starting point for challenging the State’s case.

Denuna & Westbrook, PLLC represents individuals facing DWI charges throughout Houston and the Gulf Coast region, including Harris County, Fort Bend County, Montgomery County, Galveston County, Brazoria County, and Waller County.