Challenging Field Sobriety Tests After DWI in St. Louis, MO

You were pulled over in St. Louis, and the officer asked you to perform field sobriety tests. Maybe you stumbled during the walk-and-turn, lost your balance during the one-leg stand, or had trouble following instructions during the horizontal gaze nystagmus test. Now you’re facing DWI charges, and the officer’s report claims these tests prove you were intoxicated.

Here’s what you need to know: field sobriety tests are notoriously unreliable, and they can be challenged effectively in court.

At Rose Legal Services, DWI is our most common type of case and makes up 40% of our caseload. All of our attorneys have done the same DWI training as law enforcement officers, which means we know exactly how these tests are supposed to be administered and scored. And we know when officers don’t follow proper procedures.

What Are Field Sobriety Tests?

Field sobriety tests (FSTs) are physical and cognitive exercises that police officers use during traffic stops to evaluate whether a driver is impaired by alcohol or drugs. In Missouri, officers typically administer three standardized tests endorsed by the National Highway Traffic Safety Administration (NHTSA):

Horizontal Gaze Nystagmus (HGN) Test

The officer holds a pen, small object, or even uses the tip of their finger about 12-15 inches from the driver’s face and moves it horizontally. The officer watches the subject’s eyes for involuntary jerking movements (nystagmus) that can indicate intoxication. The officer looks for:

• Lack of smooth pursuit as their eyes follow the object
• Distinct and sustained nystagmus at maximum deviation (when the subject’s eyes are moved as far to the side as possible)
• Onset of nystagmus before 45 degrees with some white showing

Walk-and-Turn Test

A person is instructed to walk heel-to-toe along a straight line for nine steps, turn around using small steps, and return heel-to-toe for nine steps. The officer watches for eight specific indicators of impairment, including:

• Cannot keep balance while listening to instructions
• Starts before instructions are complete
• Stops while walking
• Doesn’t touch heel-to-toe
• Steps off the line
• Uses arms for balance
• Makes improper turn
• Takes wrong number of steps

One-Leg Stand Test

A person is told to stand on one leg with the other foot approximately six inches off the ground and count aloud by thousands (one thousand-one, one thousand-two, etc.) for 30 seconds. The officer watches for four indicators:

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

Officers often use these test results as evidence of intoxication when making DWI arrests and when testifying in court.

Why Field Sobriety Tests Are Unreliable

Despite law enforcement’s reliance on these tests, field sobriety tests are far from scientifically reliable indicators of intoxication.

Even Sober People Fail These Tests

Studies show that even sober individuals frequently fail field sobriety tests. The NHTSA’s own research indicates that:

• The HGN test is only about 77% accurate in detecting intoxication
• The walk-and-turn test is only about 68% accurate
• The one-leg stand test is only about 65% accurate

These accuracy rates mean that significant numbers of sober people “fail” these tests, leading to wrongful DWI arrests.

Physical Conditions Affect Performance

Numerous physical conditions and characteristics affect field sobriety test performance that have nothing to do with intoxication:

• Age: People over 65 have greater difficulty with balance tests regardless of sobriety.
• Weight: Individuals more than 50 pounds overweight struggle with balance tests.
• Injuries and medical conditions: Back problems, knee injuries, ankle problems, inner ear disorders, neurological conditions, and many other medical issues affect balance and coordination.
• Footwear: High heels, boots, flip-flops, or any footwear other than flat, laced shoes makes these tests more difficult.
• Physical fitness: People who are out of shape or have sedentary lifestyles naturally struggle with physical coordination tests.

Environmental Factors Matter

The conditions under which tests are administered significantly affect performance:

• Uneven surfaces: Gravel, grass, sloped pavement, or cracked roads make balance tests nearly impossible
• Poor lighting: Nighttime testing on dark roadsides creates visibility issues
• Weather conditions: Rain, cold, wind, or extreme heat affects performance
• Traffic: Cars passing nearby create distractions and safety concerns
• Officer intimidation: The stress of performing tests with a police officer watching, traffic passing, and potential arrest looming causes anxiety that affects performance

Officers Frequently Administer Tests Improperly

NHTSA has established specific, standardized procedures for administering field sobriety tests. These procedures must be followed exactly for results to be considered valid. However, officers frequently:

• Give incorrect or incomplete instructions
• Demonstrate tests improperly
• Fail to ask about medical conditions or injuries before testing
• Administer tests on unsuitable surfaces
• Don’t allow proper time for each phase of testing
• Miscount clues or indicators of impairment
• Score tests subjectively based on their assumption that the driver is intoxicated

When officers deviate from standardized procedures, test results become even less reliable.

How to Challenge Field Sobriety Tests in a DWI Case

An experienced DWI attorney knows multiple strategies for challenging field sobriety test evidence.

Challenge the Officer’s Training and Certification

Officers must receive proper training in administering standardized field sobriety tests. An attorney should investigate:

• Whether the officer completed NHTSA-approved training
• When the officer was last certified
• How many times the officer has administered these tests
• Whether the officer’s training is current

If the officer lacks proper training or certification, their field sobriety test testimony becomes much less credible.

Challenge Administration of the Tests

An attorney should obtain all video evidence (dashcam and bodycam) and carefully review how the officer administered the tests. We look for:

• Improper instructions: Did the officer give complete, correct instructions as required by NHTSA standards?
• Improper demonstrations: Did the officer properly demonstrate each test before asking them to perform it?
• Failure to ask about medical conditions: Did the officer inquire about injuries, medical conditions, age, or weight that might affect performance?
• Unsuitable testing location: Was the surface uneven, sloped, wet, or otherwise inappropriate?
• Environmental factors: Were lighting, weather, and other conditions suitable for testing?
• Improper scoring: Did the officer correctly count and record clues, or did they subjectively decide the person “failed”?

Any deviation from standardized procedures undermines the reliability of test results and provides grounds for challenging the evidence.

Present Evidence of Medical Conditions or Physical Limitations

An attorney should gather evidence of any medical conditions, injuries, or physical characteristics that affected test performance:

• Medical records documenting back problems, knee injuries, or other relevant conditions
• Prescription medications that affect balance or coordination
• Documentation of age and weight
• Evidence of physical fitness level
• Records of inner ear problems or neurological conditions
• Other medical conditions that cause nystagmus

This evidence explains poor test performance without requiring intoxication.

Challenge the Scientific Reliability of Field Sobriety Tests

An attorney can present expert testimony challenging the scientific validity of field sobriety tests:

• Studies showing high false positive rates
• Research demonstrating that sober people frequently fail these tests
• Evidence that subjective scoring by officers introduces bias
• Scientific literature questioning whether these tests actually measure impairment

This evidence can convince judges or juries to give little weight to field sobriety test results.

Use Video Evidence to Contradict Officer Testimony

Dashcam and bodycam footage often contradict the officer’s written report or testimony. An attorney will carefully review all video evidence looking for:

• Test performance that appears better than the officer’s report suggests
• Evidence that they followed instructions correctly
• Environmental factors the officer didn’t mention
• Their physical condition and demeanor (appearing alert, speaking clearly, moving normally)
• Officer statements suggesting bias or predetermined conclusions

Video evidence frequently undermines the prosecution’s case by showing the person performed better than the officer claimed.

Present Alternative Explanations for Poor Performance

Beyond medical conditions, other explanations for poor field sobriety test performance include:

• Nervousness and anxiety: Being pulled over by police, facing arrest, and performing tests on a busy roadside naturally cause stress that affects coordination.
• Fatigue: Being tired affects balance and coordination without any alcohol involvement.
• Confusion: Complex instructions given quickly in stressful situations cause confusion, especially for people with learning disabilities or language barriers.
• Footwear and clothing: Inappropriate shoes or restrictive clothing makes physical tests difficult.
• Distraction: Traffic noise, flashing lights, and other distractions affect concentration during tests.

Why the Walk-and-Turn and One-Leg Stand Are Especially Vulnerable to Challenge

The physical agility tests (walk-and-turn and one-leg stand) are particularly unreliable and subject to challenge because:

They’re Designed to Be Difficult

These tests are divided attention exercises designed to be challenging even for sober people. They require:

• Listening to and remembering complex instructions
• Performing physical tasks while simultaneously counting or following specific patterns
• Maintaining unusual positions that humans don’t naturally adopt
• Performing under stress and observation

The tests aren’t measuring normal human capabilities — they’re measuring performance on arbitrary, difficult tasks that many sober people cannot complete successfully.

Small Deviations Are Counted as “Clues”

Officers are trained to look for specific “clues” of impairment, but many of these clues are minor deviations that don’t necessarily indicate intoxication:

• Raising arms more than six inches from sides for balance
• Stepping off a real or imaginary line by even a small amount
• Touching heel-to-toe with even a slight gap
• Taking eight or ten steps instead of exactly nine

These minor variations in performance are counted as evidence of intoxication, even though sober people frequently exhibit these same behaviors.

Officer Subjectivity Plays a Major Role

Despite NHTSA’s attempt to standardize these tests, significant subjectivity remains in how officers score them. Officers who already suspect intoxication tend to see more “clues” in the driver’s performance than officers without preconceived conclusions.

The HGN Test Has Unique Vulnerabilities

While the HGN test is considered more reliable than the physical agility tests, it still has significant weaknesses:

The Driver Can’t See What the Officer Sees

Unlike the walk-and-turn and one-leg-stand tests, where video shows the driver’s performance, no video can definitively prove or disprove the officer’s observations about involuntary eye movements. This creates credibility questions:

• Did nystagmus actually occur, or did the officer simply claim it did?
• Was the nystagmus caused by alcohol, or by other factors?
• Did the officer properly administer the test according to NHTSA standards?

Many Conditions Cause Nystagmus

Horizontal gaze nystagmus can result from numerous causes other than alcohol intoxication:

• Certain medications (particularly seizure medications and some antidepressants)
• Neurological disorders
• Inner ear problems
• Head injuries
• Eye conditions
• Fatigue
• Caffeine consumption

An attorney can present evidence of alternative causes for any observed nystagmus.

Improper Administration Invalidates Results

The HGN test must be administered precisely according to NHTSA standards:

• The stimulus must be held 12-15 inches from the driver’s face
• The stimulus must be moved smoothly at specific speeds
• Each eye must be tested separately through specific phases
• The officer must check for equal tracking, equal pupil size, and resting nystagmus first

Officers frequently rush through HGN testing or fail to follow proper procedures, invalidating the results.

Hiring an Attorney Who Knows Field Sobriety Tests

Successfully challenging field sobriety tests requires an attorney with specialized knowledge and training.

We Have the Same Training as Law Enforcement

At Rose Legal Services, all of our attorneys have done the same DWI training as law enforcement officers. This means we know:

• Exactly how field sobriety tests should be administered
• What clues officers are trained to look for
• How officers are taught to score tests
• What deviations from standardized procedures invalidate results

This specialized knowledge allows us to identify and challenge improper administration that other attorneys might miss.

We Know How to Use Expert Witnesses

In appropriate cases, we retain expert witnesses who can testify about:

• The scientific unreliability of field sobriety tests
• Proper administration procedures and how the officer deviated from them
• Medical conditions affecting test performance
• Alternative explanations for poor performance or observed nystagmus

Expert testimony can be powerful in convincing judges and juries to discount field sobriety test evidence.

We Have Extensive DWI Trial Experience

DWI is our most common type of case and makes up 40% of our caseload. We’ve challenged field sobriety tests in hundreds of cases. We know what arguments work, what evidence is most persuasive, and how to effectively cross-examine officers about their testing procedures.

We have handled thousands of cases in Missouri courts, so we’re very familiar with those prosecutors and judges. We know which judges are skeptical of field sobriety tests and how to present challenges most effectively in each courtroom.

Don’t Let Unreliable Field Sobriety Tests Convict You

If you’ve been arrested for DWI in St. Louis based on field sobriety test results, those tests can be challenged. Poor performance on these tests doesn’t necessarily prove intoxication — it may simply prove that these tests are unreliable, that you have medical conditions affecting performance, or that the officer administered them improperly.

We offer confidential consultations where we’ll review the circumstances of your arrest, examine how field sobriety tests were administered, and explain how we can challenge this evidence in your case.

Your defense starts with a conversation.