Understanding Field Sobriety Tests and Their Role in a Criminal Case

When a police officer suspects that a driver may be impaired by alcohol or drugs, the officer may ask the driver to perform a series of roadside exercises known as field sobriety tests. These tests can become an important part of a DWI or DUI investigation, but their results are not automatically proof that a person was intoxicated.

If you are facing a criminal charge involving suspected impaired driving, it helps to understand what field sobriety tests measure, how officers use them, and how the circumstances surrounding the tests may affect your case.

What Are Field Sobriety Tests?

Field sobriety tests are physical and mental exercises that officers use to look for possible signs of impairment. They often take place on the side of the road after a traffic stop.

The National Highway Traffic Safety Administration, commonly known as NHTSA, has developed three standardized field sobriety tests:

• Horizontal Gaze Nystagmus

• Walk-and-Turn

• One-Leg Stand

Officers may also ask drivers to complete nonstandard exercises, such as counting backward, reciting part of the alphabet, touching a finger to the nose, or standing with their feet together.

The standardized tests receive more formal training and have established administration procedures. Even so, the way an officer conducts the test can become an issue in a criminal case.

Horizontal Gaze Nystagmus Test

During the Horizontal Gaze Nystagmus test, an officer asks you to follow a small object, such as a pen or fingertip, with your eyes while keeping your head still.

The officer looks for involuntary eye movement called nystagmus. Alcohol consumption can affect eye movement, but other factors may also cause or influence nystagmus.

Officers typically look for six possible clues, three in each eye. These may include:

• Lack of smooth pursuit

• Distinct jerking when the eye reaches maximum deviation

• Jerking that begins before the eye reaches a specified angle

The test must be administered under appropriate conditions. Lighting, flashing emergency lights, moving traffic, medical conditions, certain medications, and the officer’s technique may affect what the officer observes.

A defense attorney may examine whether the officer positioned the stimulus correctly, moved it at the proper speed, checked for equal pupil size, or accounted for other possible causes of unusual eye movement.

Walk-and-Turn Test

The Walk-and-Turn test asks you to place one foot directly in front of the other, usually along a real or imaginary straight line. You may be instructed to take nine heel-to-toe steps, turn in a specific manner, and take nine steps back.

The officer may watch for several clues, including whether you:

• Start before the instructions are finished

• Have trouble maintaining balance

• Stop while walking

• Miss heel-to-toe contact

• Step off the line

• Use your arms for balance

• Make an improper turn

• Take the wrong number of steps

Those observations can sound significant in a police report, but context matters.

Consider a driver who performs the test at 1:00 a.m. on a sloped roadside while wearing dress shoes. Another driver may have an old knee injury or balance condition. Those circumstances could affect performance even if alcohol played no role.

Video from the officer’s body camera or patrol vehicle can be especially important. It may show whether the testing surface was level, whether instructions were clear, and whether your actual performance matches the officer’s written description.

One-Leg Stand Test

For the One-Leg Stand test, an officer typically asks you to raise one foot several inches off the ground and count aloud while keeping your arms at your sides.

The test usually lasts about 30 seconds.

An officer may look for clues such as:

• Swaying

• Using your arms for balance

• Hopping

• Putting your foot down

Physical limitations can affect this test. Age, weight, footwear, injuries, neurological conditions, fatigue, anxiety, and uneven pavement may all influence your ability to balance.

If you have a documented injury or medical condition that affects your legs, back, balance, or coordination, that information may become relevant when your attorney reviews the evidence.

Field Sobriety Tests Do Not Directly Measure Blood Alcohol Concentration

A field sobriety test does not produce a blood alcohol concentration, or BAC, reading.

A breath test or blood test may provide a numerical alcohol concentration. Field sobriety tests instead depend heavily on an officer’s observations of your behavior and physical performance.

For example, a breath test may generate a BAC result such as 0.08 percent. A Walk-and-Turn test does not provide a comparable number. The officer records whether certain clues were observed and may use those observations as part of the decision to make an arrest.

That distinction can matter when the prosecution later argues that your roadside performance supports an impairment charge.

How Officers Use the Tests During a DWI Investigation

Field sobriety testing usually occurs after an officer has already made contact with you and believes additional investigation is appropriate.

The officer may consider several observations together, such as:

• Driving behavior before the stop

• The smell of alcohol

• Red or watery eyes

• Speech patterns

• Statements about alcohol consumption

• Difficulty locating documents

• Performance on field sobriety tests

• Breath or blood test results

One weak observation may become more significant when combined with several others. The defense can examine each observation separately and then assess whether the overall evidence actually supports the officer’s conclusions.

Can You Refuse Field Sobriety Tests?

The rules surrounding field sobriety testing and refusal can vary by state. Field sobriety exercises are also legally different from chemical testing requirements involving breath or blood samples.

You should not assume that the rules governing one type of test automatically apply to another.

If you are currently facing charges, an attorney can review the law that applies in your jurisdiction and determine whether the officer properly requested and conducted each test.

For more information about selecting legal representation after a DUI or DWI arrest, you can review this guide on choosing a DUI or DWI lawyer.

Can Field Sobriety Test Results Be Challenged?

Yes. Field sobriety test evidence may be challenged in several ways depending on the facts of the case.

Your attorney may review whether the officer received proper training, followed standardized procedures, gave understandable instructions, and conducted the test under suitable conditions.

Poor Testing Conditions

Roadside testing conditions are rarely controlled.

Rain, darkness, traffic, gravel, road slope, poor lighting, extreme temperatures, and passing vehicles can interfere with your ability to follow instructions or maintain balance.

A video showing you standing on an uneven shoulder, for example, may give important context to an officer’s claim that you stepped off an imaginary line.

Medical or Physical Conditions

A person’s physical condition can affect test performance.

Common issues may include:

• Knee injuries

• Ankle injuries

• Back problems

• Inner-ear disorders

• Neurological conditions

• Vision problems

• Fatigue

• Prescription medication effects

Medical records may help establish that a physical limitation existed before the traffic stop.

Improper Instructions

Standardized tests depend on standardized procedures.

If an officer demonstrates a test incorrectly, gives incomplete instructions, changes the procedure, or fails to confirm that you understand what to do, the reliability of the observations may come into question.

Small details matter. A defense attorney may compare body-camera footage with the officer’s written report and training requirements.

Nervousness and Stress

A traffic stop can create significant stress, especially when several police vehicles, flashing lights, and passing traffic are present.

Nervousness does not automatically mean intoxication.

If an officer interprets shaking hands, rapid speech, or difficulty concentrating as evidence of impairment, the defense may argue that stress provides another reasonable explanation.

Video Evidence Can Be Critical

Police video can provide some of the strongest evidence for evaluating a field sobriety test.

A written report may state that a driver “lost balance several times.” Video may show that the driver shifted position only once. A report may say the driver failed to follow instructions, while body-camera footage may reveal that the officer interrupted or changed those instructions.

Your attorney may seek:

• Body-camera footage

• Dash-camera footage

• Police reports

• Dispatch records

• Breath-testing records

• Blood-testing records

• Officer training records

Preserving this material early can be important because some agencies have retention policies that allow certain recordings or records to be deleted after a set period.

Field Sobriety Tests and Probable Cause

Field sobriety test results may help an officer decide whether there is probable cause to make an arrest.

Probable cause does not require proof beyond a reasonable doubt. It generally involves whether the facts known to the officer provided a reasonable basis to believe an offense had occurred.

The prosecution still carries the burden required to prove the criminal charge in court.

That means an arrest based partly on field sobriety testing does not determine the final outcome of the case.

The Officer’s Report Should Be Compared With the Actual Evidence

Police reports often contain standardized language describing field sobriety test clues.

Your attorney may compare that language with video footage and other evidence.

Suppose a report states that you “used arms for balance” during a One-Leg Stand test. Video might show that your arms moved only slightly while you remained standing for the full test. That difference may affect how a judge or jury evaluates the officer’s conclusion.

The same review can apply to claims involving slurred speech, poor coordination, confusion, or difficulty following directions.

Chemical Testing May Raise Separate Issues

A criminal case involving suspected impaired driving may also include breath or blood evidence.

Those tests raise different questions, such as:

• Whether the testing device was properly maintained

• Whether required observation periods were followed

• Whether the blood sample was collected correctly

• Whether laboratory procedures were followed

• Whether the sample was stored properly

• Whether the testing instrument was calibrated

• Whether contamination or handling problems occurred

Field sobriety tests should therefore be reviewed as one part of the larger case rather than in isolation.

What You Should Do After a DWI Arrest

If field sobriety tests played a role in your arrest, preserve as much information as possible.

Write down what you remember while the events are still fresh. Include details about the road surface, weather, lighting, footwear, injuries, medications, officer instructions, and how long each test lasted.

You should also tell your attorney about any medical condition that could have affected your performance.

Avoid guessing about facts you do not remember. A police report or video can often clarify those details later.

If you need help reviewing field sobriety testing, arrest procedures, and other evidence in a Texas DWI matter, Tad Nelson & Associates DWI defense provides information about criminal defense representation.

You can also review an attorney professional profile and background when researching legal representation.

A Field Sobriety Test Is Only One Part of the Case

Field sobriety tests can influence an officer’s decision to arrest you and may later appear in the prosecution’s evidence. Their significance depends on how the tests were administered, the conditions at the scene, your physical condition, available video, and the rest of the evidence.

A poor performance does not automatically establish intoxication, and a police officer’s interpretation does not end the analysis.

If you are charged with DWI or DUI, a careful review of the traffic stop, roadside testing, chemical testing, police reports, and video evidence can help determine whether the government’s conclusions are supported by the facts