Field sobriety tests do hold up in court, but not in the way most people assume. Judges admit them as circumstantial evidence that a driver was impaired, not as proof of a specific blood alcohol concentration, and their weight at trial depends almost entirely on how the officer administered them and what the video shows. Of all the evidence a DUI prosecution typically puts forward, roadside sobriety tests are among the easiest to challenge.
What the Tests Are and What They Measure
Three roadside tests make up the Standardized Field Sobriety Test battery developed by the National Highway Traffic Safety Administration: the Horizontal Gaze Nystagmus, the Walk and Turn, and the One-Leg Stand. These are the only roadside tests backed by formal validation research, and they are the ones prosecutors rely on.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Refresher Instructor Guide
The HGN test looks for involuntary jerking of the eye as the officer moves a stimulus side to side about 12 inches from your face. The officer checks for six possible clues across both eyes, and four or more is treated as an indicator of a BAC at or above 0.08. The Walk and Turn asks you to take nine heel-to-toe steps along a line, turn, and walk nine steps back, with eight scoring clues; two triggers the impairment indicator. The One-Leg Stand requires you to stand on one foot with the other raised about six inches off the ground, counting aloud for 30 seconds, with four possible clues and the same two-clue threshold.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Refresher Instructor Guide
Officers sometimes add techniques like reciting part of the alphabet, counting backward, or touching fingertips to the thumb. NHTSA training materials state these are not scientifically validated and do not replace the three standardized tests.2National Highway Traffic Safety Administration. SFST Refresher Participant Manual That distinction matters at trial, because non-standardized tests carry less weight and are easier to exclude.
How Admissible Are the Results?
FST results are generally admissible as circumstantial evidence of impairment. They do not establish a precise BAC; that is the job of chemical tests. What this evidence does is let the officer describe your physical coordination and behavior during the stop so the prosecution can argue you were too impaired to drive safely.
For the evidence to come in, the prosecution typically has to show the officer was trained in NHTSA protocols and followed them. Significant deviations from standardized procedure give the defense grounds to challenge admissibility. A judge decides whether the jury hears the FST evidence at all, and even when it is admitted, it does not prove guilt by itself. The jury weighs it against driving behavior, speech, appearance, chemical test results, and anything else in the record.
The HGN Test Faces a Higher Bar
The HGN often gets treated differently from the other two. Because it involves interpreting an involuntary physiological response rather than watching someone walk or balance, many courts classify it as scientific evidence subject to the Frye or Daubert standards for reliability. Under Frye, the prosecution must show the science has gained general acceptance in the relevant scientific community. Under Daubert, the trial judge acts as a gatekeeper for scientific validity.3National Highway Traffic Safety Administration. Horizontal Gaze Nystagmus: The Science and the Law
How this plays out varies. In some states, the highest court has already found HGN testing reliable, so officers can testify about the results without an expert witness. In others, the prosecution must bring in expert testimony every time to lay the scientific foundation. Several states require the testifying officer to qualify as an expert specifically on HGN administration.3National Highway Traffic Safety Administration. Horizontal Gaze Nystagmus: The Science and the Law The Walk and Turn and One-Leg Stand usually face a lower hurdle, because most courts treat them as straightforward observations of physical coordination.
What Video Does to the Evidence
Dashboard and body camera footage has become one of the most powerful factors in how FST evidence plays in court. When footage exists, the jury does not have to rely only on the officer’s written report and courtroom testimony. They can watch the stop and reach their own conclusion.
Video cuts both ways. It can show that you performed the tests reasonably well despite a negative assessment, that the officer gave unclear instructions, or that road conditions made the tests unfair. It can also show clear signs of impairment that undermine a defendant’s version of events. When video contradicts an officer’s report, it creates a credibility problem that juries notice. Defense attorneys request footage early, and its absence can itself become an issue if cameras should have been running.
Does Refusing the Tests Change Anything?
In most states, roadside field sobriety tests are voluntary. There is no law requiring you to walk a line or stand on one foot during a traffic stop. Implied consent laws apply to chemical tests like a breathalyzer or blood draw after arrest, not to the physical tests before arrest. Refusing a chemical test triggers automatic consequences like license suspension in most states; refusing the roadside tests generally does not.
That does not mean refusal is cost-free. Officers can still arrest you based on driving pattern, slurred speech, odor of alcohol, or your appearance. Some courts allow prosecutors to tell the jury you declined the tests, and jurors may read that as an attempt to hide impairment. Whether the refusal itself can be admitted as evidence of consciousness of guilt varies by jurisdiction.
Fifth Amendment and Miranda at the Roadside
Courts have overwhelmingly held that physical performance on these tests is not testimonial, so the Fifth Amendment’s self-incrimination protection does not apply to the act of walking or balancing. In Pennsylvania v. Muniz, the U.S. Supreme Court reviewed a case involving videotaped sobriety test performance and noted that requiring a driver to perform physical tests does not violate the privilege against self-incrimination because the evidence produced is physical rather than testimonial.4Justia Law. Pennsylvania v. Muniz, 496 U.S. 582 (1990)
Things you say during the tests are a different matter. Statements can be admissible even without Miranda warnings as long as the officer was not asking questions designed to elicit incriminating responses; the reasoning is that standard FST instructions are not custodial interrogation. If an officer starts asking pointed questions during the tests, any answers may be challenged as obtained in violation of Miranda.4Justia Law. Pennsylvania v. Muniz, 496 U.S. 582 (1990)
What Can Undermine the Results in Court
This is where most FST-based defenses succeed or fail. The tests were designed for controlled conditions and assume a healthy, average-weight person on a flat, well-lit surface. Real traffic stops almost never match those assumptions.
Officer Errors
The single most effective challenge is showing the officer did not follow NHTSA protocols. Giving incorrect instructions, failing to demonstrate the test, rushing the HGN stimulus movement, or miscounting clues can all undermine the results. Officers are trained to hold the stimulus at maximum deviation for a minimum of four seconds during the HGN test. If dashcam footage shows them whipping through it in two seconds, the entire HGN result is compromised. The scoring is also more subjective than it sounds. Whether someone is “swaying” on the One-Leg Stand involves a judgment call, and two officers watching the same performance can disagree.
Environmental Conditions
Uneven pavement, gravel shoulders, sloped roads, poor lighting, rain, wind, and cold temperatures can make the tests harder for anyone, sober or not. Flashing emergency lights are especially disruptive during the HGN test because they create visual distractions that can affect eye movement. Heavy traffic passing close by adds noise and anxiety. A poor testing location gives the defense an argument that the environment contaminated the results.
Medical Conditions and Medications
The HGN is particularly vulnerable here. Nystagmus has dozens of causes unrelated to alcohol. Inner ear disorders like benign positional vertigo, labyrinthitis, and Meniere disease can produce nystagmus. So can neurological conditions including multiple sclerosis, stroke, and brain injuries. Anti-seizure medications like phenytoin and other sedating drugs are known to cause it as well.5MedlinePlus. Nystagmus
The physical tests have their own medical vulnerabilities. Back injuries, knee problems, leg injuries, and foot conditions can make the Walk and Turn and One-Leg Stand difficult regardless of sobriety. Age, weight, and fatigue all affect balance. The NHTSA scoring sheet itself acknowledges this by providing alternate tests for individuals who are more than 50 pounds overweight or over 65 years old. Anxiety alone can cause trembling, rushed movements, and difficulty following multi-step instructions, each of which an officer might score as a clue.
Accuracy Numbers Cut Both Ways
NHTSA’s own validation research is frequently cited at trial. The 1998 San Diego study found officers made correct arrest-or-release decisions 91 percent of the time when using all three tests together at the 0.08 threshold.6U.S. Department of Justice. Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent Defense attorneys point out what “accuracy” means in context: the study measured whether officers made correct arrest decisions, not whether the tests themselves produce reliable physical results. A 79 percent accuracy rate on the Walk and Turn means roughly one in five people were misclassified. Earlier NHTSA laboratory research found even lower individual rates.2National Highway Traffic Safety Administration. SFST Refresher Participant Manual
Drug Cases Are a Weaker Spot for the Prosecution
The three standardized tests were developed and validated for alcohol. Their usefulness for drug impairment, especially cannabis, is far less established. A 2023 randomized clinical trial published in JAMA Psychiatry gave cannabis users either THC or a placebo and had officers administer field sobriety tests. Officers classified 81 percent of the THC group as impaired, but they also classified 49 percent of the placebo group as impaired. Officers suspected 99 percent of FST-impaired participants had received THC, regardless of whether they had.7PubMed Central. Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial
The researchers concluded that FSTs are useful as supporting evidence but do not provide strong objective evidence of THC-specific impairment. Nearly half the sober placebo group failed. At least one court has already concluded there is no scientific agreement on whether these tests reliably indicate marijuana intoxication.7PubMed Central. Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial
How a Defense Attorney Attacks FST Evidence
If field sobriety tests are a key part of the prosecution’s case against you, the defense usually starts with documents. A defense attorney will request the officer’s training and certification records to verify completion of NHTSA-approved SFST training and current recertification. Outdated training or lapsed certification can undermine the officer’s credibility as a witness.
The SFST scoring sheet the officer filled out during the stop is another critical document. It records each clue the officer observed and whether you met the decision-point thresholds. Comparing what the officer wrote on the scoring sheet against what the dashcam or body camera footage actually shows is one of the most effective defense tactics. Discrepancies between the two make the officer’s testimony look unreliable.
Medical records matter as well. Any documented condition affecting balance, coordination, vision, or neurological function can explain FST clues that have nothing to do with impairment. Prescription records can be relevant if you take anything that causes nystagmus or affects motor skills.
Where FSTs Sit in the Overall Case
Field sobriety tests serve two functions, and the difference matters. The first is establishing probable cause for arrest. An officer needs a legal basis to believe you are driving impaired before they can arrest you and require a chemical test. Poor FST performance combined with other observations gives them that basis.
The second is as trial evidence, and this is where FST evidence is rarely strong enough to carry a conviction by itself. It works best for the prosecution when it lines up with chemical test results and the officer’s other observations. When FST evidence stands alone or conflicts with other evidence, it becomes vulnerable to challenge. A skilled defense attorney can turn marginal FST performance into reasonable doubt when the conditions were poor, the officer made procedural mistakes, or the video tells a different story than the police report.