DUI Field Sobriety Tests: Accuracy, Refusal, and Defenses

DUI field sobriety tests are the roadside physical and mental exercises an officer uses to decide whether to arrest you for impaired driving. Three of them — the eye-tracking test, the walk-and-turn, and the one-leg stand — are standardized by the National Highway Traffic Safety Administration, and in most states they are voluntary. Field research using experienced officers found the three-test battery pointed to the correct arrest decision at a 0.08 BAC roughly 91 percent of the time.¹ That still leaves about one in eleven decisions wrong, and knowing what the officer is counting — and what can trip you up when you are sober — is the point of this guide.

The Three Standardized Tests

NHTSA, together with the International Association of Chiefs of Police, approves exactly three roadside tests as scientifically validated. Any other exercise an officer asks you to perform falls outside the validated battery.¹

Horizontal Gaze Nystagmus

The officer holds a stimulus — pen, penlight, or fingertip — about 12 to 15 inches from your nose and slightly above eye level, then moves it slowly side to side while you follow it with your eyes and keep your head still. The test checks for nystagmus, an involuntary jerking of the eyeball that becomes more pronounced when alcohol or certain drugs depress the central nervous system.¹

Each eye is checked for three clues: whether it tracks the stimulus smoothly or jerks, whether it bounces visibly when held at the far edge of your vision for at least four seconds, and whether the jerking begins before the eye reaches a 45-degree angle from center.² The officer may also raise the stimulus straight up and hold it to check for vertical nystagmus, which can indicate a particularly high level of impairment.

Walk and Turn

You stand heel-to-toe, left foot behind right, arms at your sides while the officer explains the exercise. Then you take nine heel-to-toe steps along a real or imaginary line, pivot with a series of small steps, and take nine heel-to-toe steps back, counting out loud and watching your feet.¹

The officer watches for eight clues: losing balance during instructions, starting too soon, stopping mid-walk, failing to touch heel to toe, stepping off the line, raising arms for balance, turning improperly, and taking the wrong number of steps. The exercise is built as a divided-attention task, forcing you to listen, remember, and move at once.

One-Leg Stand

You raise one foot about six inches off the ground, keeping both legs straight, look at your elevated foot, and count aloud — “one thousand one, one thousand two” — until told to stop. The officer silently times thirty seconds. NHTSA’s original research found many impaired subjects can hold the position for 25 seconds, but few reach 30.¹ The four clues: swaying, raising arms six or more inches from the body, hopping, and putting the foot down before time is up.

What Score the Officer Is Counting Toward

Officers do not grade you on feel. Each test has a specific number of clues that, according to NHTSA research, statistically predicts a BAC at or above 0.08: four or more out of six for the eye test, two or more out of eight for the walk and turn, and two or more out of four for the one-leg stand.¹ If you cannot complete the walking or balance test at all, that also counts as meeting the threshold.

Those cut-points are not pass-fail grades. Hitting the threshold does not prove you were over the legal limit; it means the research population showing that many clues was more likely than not to be at 0.08 or higher. The distinction matters if your case reaches a courtroom.

How Accurate the Tests Really Are

Original laboratory research put the eye-tracking test at 77 percent accuracy on its own, the walk and turn at 68 percent, and the one-leg stand at 65 percent. Later field studies using experienced officers reported 88 percent, 79 percent, and 83 percent respectively.¹ Combined, the three-test battery produced correct arrest decisions roughly 91 percent of the time at the 0.08 threshold.

Those figures also assume the tests were administered exactly as trained. That condition does not always hold on a gravel shoulder at one in the morning.

Why a Sober Person Can Fail

The real vulnerability of roadside testing is that many things other than alcohol produce the exact clues officers count.

Medical Conditions

Nystagmus occurs in people who have never had a drink. Cerebellar disease, inner ear disorders, multiple sclerosis, and brainstem conditions all produce eye jerking that looks identical to the alcohol-induced version. Even healthy people often show a small-amplitude jerking when their eyes are held past 40 degrees, and separating that physiological nystagmus from an impairment indicator by eye alone is extremely difficult.³

The balance tests have their own built-in problem. The NHTSA manual itself acknowledges that people over 65, anyone with back, leg, or inner ear problems, and people who are 50 or more pounds overweight may have trouble performing the walk-and-turn and one-leg stand regardless of sobriety.¹ The manual does not require officers to ask about medical history before the tests begin.

Medications

Prescription drugs are a major source of false-positive nystagmus. Anticonvulsants such as carbamazepine and zonisamide are strongly associated with drug-induced eye movement disorders, including nystagmus. Anti-anxiety medications like lorazepam and clobazam, antidepressants like sertraline and fluoxetine, and even antivirals like acyclovir have been documented to produce the same involuntary jerking.⁴ A sober person on certain seizure or psychiatric medication can present textbook clues that an officer, scoring by the rubric, counts against them.

Surface, Lighting, and Footwear

The NHTSA manual says the walking and balance tests should be done on a “reasonably dry, hard, level, non-slippery surface” whenever possible, and recommends either relocating or using only the eye test when such a surface is not available.¹ In practice, officers often conduct the full battery on gravel shoulders, sloped parking lots, and wet pavement. Footwear matters too. High heels or loose sandals affect balance, and the manual lets you remove them, though not every officer offers the option.

Non-Standardized Tests

Officers sometimes add exercises outside the validated battery. These lack the controlled research backing of the three standardized tests, so they carry less evidentiary weight, though officers still use them to build a broader impairment picture.

The finger-to-nose test asks you to tilt your head back, close your eyes, and touch the tip of your nose with the specified finger. The Rhomberg balance test has you stand with feet together, head back, eyes closed, and estimate when thirty seconds have passed. Both check proprioception, the body’s sense of its own position. Verbal tasks are also common — reciting part of the alphabet without singing, or counting backward between two numbers. Because these exercises have no standardized scoring, the officer’s subjective judgment weighs more heavily, which also makes the results easier to challenge later.

Your Right to Refuse

In most states, roadside field sobriety tests are entirely voluntary. You are generally not required to perform any of the physical exercises an officer requests during a traffic stop. The officer needs only reasonable suspicion to ask, but asking is not the same as compelling.

Refusing comes with trade-offs. The officer can still build probable cause from other observations: alcohol odor on your breath, slurred speech, bloodshot eyes, open containers, or erratic driving. In some jurisdictions, the prosecution can tell a jury about your refusal as evidence of consciousness of guilt. At the same time, refusing denies the officer a set of scored, documented clues that are hard to rebut later. There is no universally right answer. How much you have had to drink and whether medical or environmental conditions would work against you both factor in.

There is generally no right to consult an attorney before deciding whether to perform roadside tests. The right to counsel typically attaches later, after arrest, when you face decisions about chemical testing at the station.

Implied Consent and Chemical Tests After Arrest

Field sobriety tests and chemical tests sit under completely different rules. Once an officer arrests you for DUI, implied consent laws in every state require you to submit to a breath or blood test. These laws rest on the premise that by driving on public roads, you have already agreed to chemical testing if lawfully arrested for impaired driving.

The U.S. Supreme Court drew an important line in Birchfield v. North Dakota. The Court held that the Fourth Amendment permits warrantless breath tests as a search incident to a DUI arrest but does not permit warrantless blood tests, because blood draws are too invasive to be justified without a warrant. States can impose civil penalties and license consequences for refusing a breath test, but they cannot impose criminal penalties for refusing a blood test absent a warrant.⁵

Refusing a chemical test after arrest triggers administrative penalties that vary by state but commonly include license suspension from several months to a year or more for a first offense, with longer suspensions for repeat refusals. Those suspensions typically take effect regardless of whether the DUI charge itself results in conviction. Reinstatement usually involves additional fees, and many states require high-risk auto insurance for years afterward.

Challenging the Results Later

Field sobriety evidence is not bulletproof. The NHTSA manual itself provides the framework for most successful challenges, because its specificity about proper procedure creates a clear standard against which an officer’s actual conduct can be measured.

Protocol Deviations

The manual is explicit that the validity of these tests depends on administering them “in the prescribed and standardized manner,” using “standardized clues” and “standardized criteria” for interpretation.¹ If any element changes, the manual says validity may be compromised. Did the officer move the stimulus too quickly? Hold maximum deviation for two seconds instead of four? Demonstrate the turn incorrectly? Each deviation is a point of attack.

Courts in many states treat minor timing variations as going to the weight jurors give the evidence rather than whether it comes in at all. An officer who took 11 seconds on a smooth pursuit pass instead of 16 may still testify, but defense counsel can use the shortcut to argue the results are unreliable. When an officer admits a test was administered incorrectly, courts are more likely to exclude the results outright.

Medical, Medication, and Environmental Defenses

The medical conditions and medications discussed earlier form the backbone of many successful challenges. Anticonvulsants, anti-anxiety medication, and inner ear disorders directly undermine the reliability of the eye-tracking test, and the NHTSA manual does not require officers to screen for them. A sloped or wet surface, loose gravel, or inadequate lighting undercuts the balance-dependent tests.

Video Evidence

Dashcam and bodycam footage has become one of the most powerful tools in DUI defense. Video lets a jury see what happened rather than relying only on an officer’s written report. It can show that you performed better than the report suggests, that instructions were unclear or rushed, that the testing surface was obviously unsuitable, or that the officer skipped procedural steps. The absence of video when recording equipment was available can also raise questions about the reliability of the officer’s account.

One narrower point on the eye-tracking test is worth knowing. Even where courts admit it, officers generally cannot testify that a specific number of nystagmus clues corresponds to a specific BAC. Results can show that a driver was likely impaired, but they cannot quantify the level of impairment.⁶ That limit is why prosecutors usually anchor a DUI case to the chemical test, not the roadside one.

1
National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing (SFST) Participant Manual
2
National Highway Traffic Safety Administration. Standardized Field Sobriety Testing SFST Refresher Manual
3
Journal of the Royal Society of Medicine. Mechanisms Underlying Nystagmus
4
National Center for Biotechnology Information. Pharmacovigilance Study of Drug-Induced Eye Movement Disorder
5
Justia US Supreme Court. Birchfield v. North Dakota
6
National Highway Traffic Safety Administration. Horizontal Gaze Nystagmus: The Science and The Law