When an officer suspects a driver of impairment during a traffic stop in Tennessee, the next step is often a request to step out of the car and perform a series of physical tasks on the side of the road. These are field sobriety tests, and the three that carry official standing — the horizontal gaze nystagmus test, the walk-and-turn, and the one-leg-stand — form the backbone of many DUI arrests across Davidson County and Middle Tennessee. The results become the officer’s justification for the arrest and, later, evidence the state offers at trial.
What many drivers do not know is that these tests are voluntary in Tennessee, that their accuracy rates are far below what people assume, and that they depend on precise administration that officers do not always follow. This article explains what the three standardized tests measure, where the science comes from, how reliable they are, and how a defense attorney challenges field sobriety evidence when it becomes the foundation of a DUI charge.
Where Field Sobriety Tests Come From
The three standardized field sobriety tests were developed through research sponsored by the National Highway Traffic Safety Administration, or NHTSA, beginning in the 1970s. The goal was to identify a small battery of roadside tasks that correlated with alcohol impairment and could be administered consistently by officers in the field. Out of a larger set of candidate tests, three emerged as the standardized battery: horizontal gaze nystagmus, the walk-and-turn, and the one-leg-stand.
The word “standardized” is the key to understanding both the strength and the weakness of these tests. NHTSA’s research supports the tests only when they are administered and scored exactly as the protocol requires. The validation studies measured accuracy under controlled conditions with trained administrators following the procedure to the letter. When an officer deviates from the standardized method — changes the instructions, skips a step, or scores a clue that was not properly demonstrated — the research no longer backs the result. The test becomes something other than the validated instrument NHTSA studied.
This is the central point a defense attorney returns to again and again. The tests are only as reliable as their administration, and roadside conditions — uneven pavement, poor lighting, traffic noise, weather, the driver’s footwear and physical condition — are a long way from a controlled study. The gap between the laboratory and the shoulder of Interstate 40 is where much of the defense work happens.
The Three Standardized Tests
Horizontal Gaze Nystagmus (HGN)
Nystagmus is an involuntary jerking of the eyes. The HGN test asks the driver to follow a stimulus — usually a pen or a fingertip — with their eyes while the officer watches for that jerking as the eyes move toward the outer edges of their range. The theory is that alcohol exaggerates nystagmus, so the officer looks for a set number of clues across both eyes. Of the three tests, NHTSA-sponsored research rates HGN as the most reliable, at roughly 77 percent when administered correctly.
HGN is also the test most vulnerable to challenge, because nystagmus has many causes that have nothing to do with alcohol. Fatigue, certain medications, neurological conditions, inner-ear problems, and even naturally occurring nystagmus can produce the same eye movement. The test also requires the officer to hold the stimulus at the right distance and speed and to make fine visual judgments about involuntary movement — a demanding task to perform accurately at night on the roadside.
The Walk-and-Turn
The walk-and-turn is a “divided attention” test. The driver is told to take nine heel-to-toe steps along a line, turn in a prescribed way, and walk back, all while following specific instructions and keeping their hands at their sides. The officer scores clues such as stepping off the line, starting too soon, using arms for balance, taking the wrong number of steps, or performing an improper turn. NHTSA-sponsored research puts the accuracy of the walk-and-turn at about 68 percent when administered correctly.
The test assumes a level, dry, well-lit surface and a driver who is not affected by age, weight, injury, or footwear. A driver over sixty-five, someone fifty pounds or more overweight, or a person with a back, leg, or inner-ear condition may struggle with the test for reasons unrelated to alcohol. High heels or work boots can throw off the heel-to-toe walk. NHTSA’s own materials acknowledge these limitations, which a defense attorney can raise directly.
The One-Leg-Stand
The one-leg-stand asks the driver to stand on one foot with the other raised about six inches off the ground, and to count aloud until told to stop, usually for about thirty seconds. The officer watches for swaying, hopping, putting the foot down, or using the arms for balance. Of the three standardized tests, the one-leg-stand carries the lowest accuracy rate in NHTSA-sponsored research, at roughly 65 percent when administered correctly.
Balance tests are affected by the same physical factors as the walk-and-turn, and standing on one leg for thirty seconds is difficult for many sober people, especially on an uneven roadside at night. Age, weight, injury, medical conditions, and nervousness all degrade performance. When two of the three standardized tests rest heavily on balance and coordination that vary widely among sober individuals, the combined battery is far from a definitive measure of impairment.
What the Accuracy Numbers Mean
The accuracy figures — 77 percent for HGN, 68 percent for the walk-and-turn, and 65 percent for the one-leg-stand — are worth reading carefully. These are the rates reported in NHTSA-sponsored validation research under proper administration. Even taken at face value, they mean the tests are wrong a meaningful share of the time. A test that is accurate 65 percent of the time is wrong about one time in three.
There are two more caveats. First, “accuracy” in these studies typically measured whether the tests could predict a blood alcohol concentration above a threshold — not whether a driver was in fact impaired in a way that affected their driving. Second, the numbers assume textbook administration. When the officer deviates from protocol, the reported accuracy no longer applies, and the result is a data point of unknown reliability. A defense attorney’s job is to show the jury that a number presented as scientific is nothing of the kind once the administration is examined.
This matters most when a DUI case rests on the impairment theory rather than the chemical-test number. The distinction between proving impairment and proving a per se violation runs through every DUI prosecution, and it shapes how much weight field sobriety evidence carries. Our overview of DUI per se versus impairment in Tennessee explains why the state sometimes leans on field sobriety tests and sometimes leans on a breath or blood result.
It also helps to understand what the field sobriety battery was never designed to do. The tests were built as a screening tool to help an officer decide whether there is probable cause to arrest — not as a laboratory measurement of a specific blood alcohol level or of a driver’s actual driving ability at a given moment. Treating a set of roadside clues as if it were a precise scientific gauge overstates what the research supports. A jury that understands the difference between a screening tool and a measurement is better positioned to weigh the evidence for what it is.
Non-Standardized Tests and the Preliminary Breath Test
Officers sometimes ask drivers to perform tasks that are not part of the standardized battery — reciting the alphabet from a certain letter, counting backward, touching finger to nose, or tilting the head back with eyes closed to estimate the passage of thirty seconds. None of these carry the NHTSA validation that the three standardized tests have. There is no published accuracy rate for them, no standardized scoring, and no research establishing that they correlate with impairment. A defense attorney can point out that a non-standardized task is a subjective judgment by the officer with no scientific footing at all.
A related tool is the preliminary or portable breath test, a handheld device an officer may offer at the roadside. This is distinct from the evidentiary breath test administered later at the station under the implied-consent law. The roadside preliminary test is generally used to help establish probable cause rather than to prove a specific blood alcohol number in court, and its results are treated differently from the evidentiary instrument. Like the field sobriety tests, the roadside portable device has limitations a defense attorney can examine, from calibration to the conditions of use.
The practical point is that not everything an officer asks a driver to do carries the same weight. The three standardized tests have research behind them, however qualified; the ad hoc tasks and the portable roadside device stand on weaker ground. Sorting the validated from the unvalidated is part of how a defense attorney takes apart the state’s roadside evidence.
You Can Decline Field Sobriety Tests in Tennessee
Field sobriety tests are voluntary in Tennessee. Unlike a breath or blood test after an arrest, which is governed by the state’s implied-consent law and carries an automatic license penalty for refusal, there is no separate statutory penalty for declining to perform field sobriety tests. A driver who is asked to step out and perform the walk-and-turn or the one-leg-stand can decline to do so.
This is a distinction drivers frequently miss. Implied consent, which attaches to chemical testing, is a different legal regime with its own consequences. The roadside physical tests sit outside that framework. Declining them does not carry the automatic license revocation that refusing a post-arrest breath or blood test does.
That said, declining is not consequence-free. An officer who is denied field sobriety tests may still develop probable cause from other observations — the odor of alcohol, slurred speech, driving behavior — and make an arrest. And a refusal can be commented on in some circumstances. What declining does accomplish is deny the state a set of roadside results that can be difficult to explain away later. This is general information about how the law is structured, not advice about what any particular driver should do in the moment; those decisions depend on the specific circumstances of a stop.
How a Defense Attorney Challenges the Tests
When a DUI case includes field sobriety evidence, a defense attorney works methodically through the administration and the surrounding conditions. The goal is to show that the results are unreliable, that they were improperly obtained, or both.
Examining the Administration
Because the tests are validated only under standardized administration, deviation is the first thing to look for. Did the officer give the instructions in the required form? Was the stimulus held at the correct distance and moved at the correct speed during HGN? Was the surface level and dry? Was the driver’s age, weight, or physical condition a factor the officer ignored? Body-camera and dash-camera footage, where it exists, lets a defense attorney compare what the officer did against the NHTSA protocol step by step.
Raising Alternative Explanations
Many clues the officer scored have innocent explanations. Nystagmus can come from fatigue or medication. Balance problems can come from an injury, a medical condition, or simple nervousness at being stopped by police at night. Footwear and roadside conditions degrade performance for sober people. Presenting these alternatives — sometimes through the driver’s medical history, sometimes through an expert — reframes the officer’s clues as ordinary human behavior rather than signs of impairment.
Connecting the Tests to the Stop and Arrest
Field sobriety tests do not exist in isolation. They follow a traffic stop that had to be supported by reasonable suspicion and precede an arrest that had to be supported by probable cause. If the stop itself was unlawful, everything that followed — including the tests — may be subject to exclusion. Our overview of search and seizure under the Fourth Amendment explains the constitutional limits on stops and detentions in Tennessee.
Where the stop, the detention, or the arrest was not lawful, the remedy is a motion to suppress the resulting evidence. If the tests and the observations that flowed from an unlawful stop are excluded, the state may be left without enough to proceed. The mechanics of these motions are the subject of our discussion of suppression motions in Tennessee, and they are frequently the most powerful tool available in a DUI case built on roadside testing.
Field Sobriety Evidence in Middle Tennessee Courts
In Davidson County and the surrounding counties, DUI cases regularly turn on how a jury or judge weighs field sobriety evidence against the alternative explanations a defense presents. Officers who conduct these tests are trained in the NHTSA protocol, and an attorney familiar with that same protocol can cross-examine on the specifics — the exact instructions, the exact clues, the exact scoring — rather than in generalities. The more precise the challenge, the harder it is for the state to present the results as conclusive.
Nathan Cate is a court-qualified criminal defense expert witness and defends DUI cases throughout Middle Tennessee. Field sobriety evidence is often the weakest part of a DUI prosecution once it is examined closely, and a driver who understands that the tests are voluntary and imperfect is in a far better position than one who assumes the results speak for themselves. For a broader picture of how a Nashville-area defense practice approaches these cases, see our page for the Davidson County criminal defense attorney.
What matters most is timing. Video evidence, dispatch records, and calibration logs do not last forever, and the sooner they are requested and preserved, the more a defense can do with them. A driver who has been arrested after roadside testing should treat the collection of that evidence as urgent. The clues an officer wrote down in a report are only one version of what happened; the footage and the underlying records often tell a fuller story, and that story is where a strong field sobriety challenge begins.
Frequently Asked Questions
Do I have to take field sobriety tests in Tennessee?
No. Field sobriety tests are voluntary in Tennessee. Unlike a post-arrest breath or blood test, which is governed by the implied-consent law and carries an automatic license penalty for refusal, there is no separate statutory penalty for declining to perform the roadside physical tests. An officer may still develop probable cause to arrest from other observations, but you are not legally required to attempt the walk-and-turn, the one-leg-stand, or HGN.
How accurate are field sobriety tests?
NHTSA-sponsored research reports accuracy of roughly 77 percent for HGN, 68 percent for the walk-and-turn, and 65 percent for the one-leg-stand — and only when the tests are administered exactly according to protocol. Even at those rates, the tests are wrong a meaningful share of the time, and the reported accuracy no longer applies when an officer deviates from the standardized method. The numbers are far from the near-certainty many people assume.
What are the three standardized field sobriety tests?
The three standardized tests are the horizontal gaze nystagmus (HGN) test, which observes involuntary eye movement; the walk-and-turn, which requires heel-to-toe steps along a line with a prescribed turn; and the one-leg-stand, which requires balancing on one foot while counting. These three were developed through NHTSA-sponsored research and are the only field sobriety tests with formal validation. Other roadside tasks an officer might use are not standardized.
Can medical conditions affect field sobriety test results?
Yes. Age, weight, injuries, inner-ear problems, neurological conditions, certain medications, and even nervousness can all degrade performance on the balance-based tests and produce nystagmus unrelated to alcohol. NHTSA’s own materials recognize that factors such as being over sixty-five, being significantly overweight, or having a leg or back condition affect the walk-and-turn and one-leg-stand. A defense attorney can raise these alternative explanations for the clues an officer scored.
Can field sobriety results be thrown out of court?
They can be excluded in some cases. If the traffic stop or arrest that led to the tests was unlawful, a motion to suppress can exclude the evidence that followed, including the field sobriety results. The tests can also be undermined at trial by showing improper administration or innocent explanations for the clues. Whether exclusion is available depends on the specific facts of the stop, the arrest, and how the tests were conducted.
Talk to a Nashville Criminal Defense Lawyer
If you are facing criminal charges in Tennessee, the decisions you make early in your case can shape everything that follows. Nashville criminal defense attorney Nathan Cate represents clients charged with felonies and misdemeanors throughout Davidson County and Middle Tennessee. With 53 jury trials taken to verdict and 12 outright Not Guilty acquittals, he brings courtroom experience to every stage of a case — from the first hearing through trial and appeal.
Call (615) 664-8083 to schedule a consultation, or visit the office at 222 2nd Avenue North, Suite 220, Nashville, TN 37201.