The physical signs of intoxication that police look for during a traffic stop fall into two groups: visible cues the officer catalogs through the window and from a few feet away, and performance failures on three standardized roadside tests. Red or glassy eyes, slurred speech, the smell of alcohol, and unsteady movement come first. If those cues add up, the officer moves to the Horizontal Gaze Nystagmus, Walk-and-Turn, and One-Leg Stand tests, which together correctly identify a driver at or above a 0.08% blood alcohol concentration 91% of the time.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Instructor Guide
What Officers Watch For Before Any Testing
The assessment starts the moment the officer walks up to your window. Bloodshot or glassy eyes are usually the first note in the report, because alcohol dilates the small blood vessels on the surface of the eye and leaves them visibly red. Heavy or drooping eyelids point to a depressed nervous system. Flushed skin on the cheeks or neck is another common cue, caused by the body processing ethanol.
Speech gives officers a lot to work with. Alcohol affects the muscles involved in articulation, so words come out slurred or mumbled. Volume swings between too loud and barely audible. Impaired drivers often repeat the same phrase without realizing it, which signals disrupted short-term memory. Officers also note whether a driver seems unusually talkative or is using language they probably wouldn’t choose sober. Body cameras capture all of this, and prosecutors use the footage in court.
The smell of alcohol is really the smell of alcohol vapors leaving the bloodstream through the lungs, with some reaching the officer through skin perspiration as well. A freshly consumed drink smells different from the stale, sweet scent of alcohol the body has been metabolizing for hours. The odor by itself doesn’t prove any particular level of impairment, but it confirms alcohol is in the system and gives the officer a basis to investigate further.
Coordination problems round out the picture before formal testing. Staggering, swaying while standing still, or reaching for the car to keep your balance all indicate that the brain is struggling with spatial orientation. Fine motor lapses are equally telling: fumbling with a wallet, dropping an ID, or struggling to pull keys from a pocket. These actions depend on the cerebellum, and alcohol disrupts the signaling pathways there by altering neurotransmitter activity in its cells.2National Center for Biotechnology Information. Effects of Ethanol on the Cerebellum: Advances and Prospects
The Three Standardized Field Sobriety Tests
If those observations give the officer reasonable suspicion of impairment, the next step is the Standardized Field Sobriety Test battery. The battery was developed and validated by the National Highway Traffic Safety Administration and consists of three tests. Each one is a “divided attention” task: you have to listen to instructions, remember them, and perform a physical action at the same time, the same mental juggling that driving requires. Sober people manage it without trouble. Impaired people show predictable breakdowns.
Horizontal Gaze Nystagmus
Nystagmus is an involuntary jerking of the eyeball that a person cannot control or suppress. Alcohol makes the jerking pronounced as the eyes track an object moving to the side. The officer holds a stimulus, usually a pen or small flashlight, about 12 to 15 inches from your face and slowly moves it from center toward one ear while watching your eye.3National Highway Traffic Safety Administration. Horizontal Gaze Nystagmus: The Science and The Law
There are three clues to watch for in each eye, six in total:
- Lack of smooth pursuit, where the eye bounces or stutters as it follows the stimulus instead of gliding.
- Distinct nystagmus at maximum deviation, where the eye jerks sustainably when held as far to the side as possible for four seconds.
- Onset of nystagmus before 45 degrees, meaning the jerking begins before the eye has moved halfway to the side.
Four or more clues indicate a BAC at or above 0.08%. The HGN test alone is 88% accurate at making that call.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Instructor Guide It is the one test in the battery that targets an involuntary physiological response, which is why officers and prosecutors treat it as the most reliable of the three.
Walk-and-Turn
The Walk-and-Turn has two stages. During the instruction stage, you stand heel-to-toe with your arms at your sides while the officer explains what to do. The test itself requires nine heel-to-toe steps along a straight line, a specific turn, and nine steps back. Officers score eight possible clues:4National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
- Losing balance while listening to instructions
- Starting before being told to begin
- Stopping while walking
- Failing to touch heel-to-toe
- Stepping off the line
- Raising arms for balance
- Making an improper turn
- Taking the wrong number of steps
Two or more clues indicate a BAC at or above 0.08%, correctly classifying about 79% of subjects. What trips up most impaired people is the instruction stage itself. Standing still in a heel-to-toe position while listening to and remembering a multi-step sequence is exactly the kind of divided-attention task alcohol makes difficult.
One-Leg Stand
The One-Leg Stand requires you to raise one foot about six inches off the ground, keep the leg straight, stare at the elevated foot, and count out loud (“one thousand and one, one thousand and two…”) for 30 seconds. Officers watch for four clues: swaying while balancing, raising one or both arms more than six inches from your side, hopping on the planted foot, and putting the raised foot down before the 30 seconds are up.4National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
Two or more clues correctly classify 83% of subjects as being at or above 0.08% BAC.5National Highway Traffic Safety Administration. Standardized Field Sobriety Testing Refresher Participant Manual The counting component is a cognitive load that a sober person handles almost automatically. An impaired person tends to lose track.
How the Observations Add Up to Probable Cause
No single observation or failed test normally produces an arrest on its own. Officers look at the totality of the circumstances: driving behavior before the stop, face-to-face observations during the initial contact, and performance on the roadside tests. Erratic driving plus bloodshot eyes, the smell of alcohol, slurred speech, and two or more clues on the Walk-and-Turn is a much stronger case than any one factor by itself. When all three standardized tests are administered together, NHTSA found the combined battery is 91% accurate at correctly identifying drivers at or above 0.08% BAC. That combined accuracy is what gives courts confidence the results, paired with the officer’s other observations, establish probable cause for an arrest.
Can You Refuse the Roadside Tests?
In most states, you are not legally required to perform field sobriety tests. The roadside exercises are voluntary, and declining does not carry the automatic penalties that refusing a chemical test does. Officers rarely spell that out during a stop, and the request to “step out of the car and perform a few tests” sounds like an order.
Refusing does not mean you will be sent on your way. The officer can still arrest you based on everything else: your driving pattern, slurred speech, the smell of alcohol, and anything else observed during the encounter. In some jurisdictions, a prosecutor may argue that refusal suggests consciousness of guilt. Declining the tests removes one category of evidence but does not end the investigation.
Chemical Testing After Arrest Is Different
Breath, blood, and urine tests given after arrest operate under a separate legal framework. All 50 states have implied consent laws, meaning that by holding a driver’s license and using public roads you have already agreed to submit to chemical testing after a lawful DUI arrest. Refusing a post-arrest chemical test carries its own penalties.
The most common consequence is an automatic driver’s license suspension, typically running from several months to two years for a first refusal. Repeat refusals carry significantly longer suspensions in many states. The suspension applies regardless of whether you are ultimately convicted, because it is treated as a consequence of breaking the implied consent agreement rather than a criminal punishment.
One legal boundary matters here. In Birchfield v. North Dakota (2016), the U.S. Supreme Court held that warrantless breath tests are permitted during a DUI arrest and that states may impose criminal penalties for refusing one. The Court held that warrantless blood tests are not permitted under the same circumstances, and states cannot criminalize refusal of a blood draw without a warrant.6Justia Law. Birchfield v. North Dakota, 579 U.S. (2016) States can still impose civil penalties like license suspension for blood test refusal, just not treat it as a separate crime.
A preliminary breath test offered at the roadside before arrest is a separate category. In many states you can decline a roadside preliminary breath screening without triggering implied consent penalties. The distinction turns on timing: implied consent obligations generally attach at the point of a lawful arrest, not during the initial roadside investigation.
Medical Conditions That Produce the Same Signs
Not every physical sign that looks like impairment comes from alcohol. This is the blind spot in the standardized tests, and where defense attorneys focus their energy.
Nystagmus has dozens of causes unrelated to alcohol. Inner ear disorders like benign paroxysmal positional vertigo and Ménière’s disease can produce it. So can neurological conditions including multiple sclerosis, stroke, and brain tumors.7Cleveland Clinic. Nystagmus Prescription medications are another common culprit, particularly anticonvulsants like phenytoin and carbamazepine, as well as lithium and certain sedatives. Someone taking legally prescribed seizure medication can show pronounced nystagmus on the HGN test with zero alcohol in their system.
Diabetic emergencies create an especially convincing false picture. When blood sugar drops dangerously low, a person can become confused, combative, and slur their words, all signs officers associate with intoxication. Diabetic ketoacidosis, which occurs with high blood sugar, causes the body to produce ketones and exhale them through the lungs. That breath has a sweet, acetone-like odor that is frequently mistaken for alcohol. Emergency medical providers are trained to check for medical alert jewelry and insulin pumps before assuming someone is intoxicated. A roadside officer may not make the same assessment.
Physical disabilities, inner ear surgery, head injuries, and extreme fatigue can also affect balance and coordination in ways that resemble impairment during the Walk-and-Turn or One-Leg Stand.
If you have a medical condition that can produce these signs, telling the officer at the time of the stop creates a record. Telling your attorney gives them a concrete basis for challenging the results. The standardized tests are validated tools, but they were validated on populations without these conditions, and that gap is where many challenges succeed.