The SFST, or Standardized Field Sobriety Test, is a battery of three physical tests police officers use during traffic stops to determine whether a driver is impaired by alcohol or drugs. Developed and validated by the National Highway Traffic Safety Administration (NHTSA), the battery includes an eye test, a walking test, and a balance test. When all three are administered together following standardized procedures, officers make correct arrest decisions in over 91% of cases at the 0.08% blood alcohol level.
The Three Tests in the SFST Battery
Each test in the battery targets a different set of abilities that alcohol impairs: involuntary eye movement, the ability to follow instructions while performing physical tasks, and balance. Officers are trained to administer them in a specific order, using scripted instructions and demonstrations. Skipping steps or changing the procedure can compromise the results.
The three tests are the Horizontal Gaze Nystagmus (HGN) test, the Walk and Turn test, and the One Leg Stand test. The HGN is the most scientifically grounded of the three because it measures an involuntary response you cannot consciously control. The other two are “divided attention” tests, meaning they require you to listen to instructions, remember them, and execute physical movements at the same time, something that becomes significantly harder when impaired.
How the Eye Test Works
Nystagmus is an involuntary jerking of the eyes that becomes more pronounced when a person has alcohol or certain drugs in their system. During the HGN test, the officer holds a small stimulus like a pen or penlight about 12 to 15 inches from your nose, slightly above eye level, and slowly moves it side to side. You follow the stimulus with your eyes while keeping your head still.
The officer checks each eye independently for three specific clues. First, whether your eyes follow the stimulus smoothly or jerk as they track it, like a windshield wiper skipping across a dry windshield. Second, whether your eye continues to jerk when held at the far edge of your gaze for at least four seconds. Third, whether the jerking begins before your eye reaches a 45-degree angle from center, which correlates with a blood alcohol concentration above 0.08%. With three clues possible per eye, the maximum score is six. Four or more clues indicates impairment.
Alcohol disrupts eye movement control by depressing the central nervous system. It interferes with the brain’s main inhibitory chemical messenger, which plays a direct role in controlling the speed and smoothness of eye movements. This is why nystagmus shows up reliably at certain alcohol levels. You cannot suppress it through effort or practice.
The Walk and Turn Test
This test begins before you take a single step. The officer asks you to stand in a heel-to-toe position, left foot on a line (real or imaginary) with your right foot placed directly in front of it, heel touching toe. You hold this position with your arms at your sides while listening to the rest of the instructions. Many people show their first signs of impairment during this instruction phase alone, either by losing balance or starting to walk before being told to begin.
Once instructed, you take nine heel-to-toe steps along the line, turn using a series of small steps while keeping your lead foot planted, and take nine heel-to-toe steps back. You must count each step out loud, keep your arms at your sides, watch your feet, and not stop once you start walking.
Officers score eight possible clues: losing balance during the instructions, starting too soon, stopping while walking, missing heel-to-toe contact by more than half an inch, stepping off the line, raising arms six inches or more for balance, turning improperly, and taking the wrong number of steps. Two or more clues indicates impairment.
The One Leg Stand Test
The simplest test to describe, but not necessarily the easiest to perform. You stand with your feet together, then raise one foot approximately six inches off the ground with your toes pointed forward. While holding that position, you count out loud (“one thousand one, one thousand two…”) for 30 seconds.
Officers watch for four clues: swaying while balancing, using arms to balance, hopping, and putting your foot down. Two or more clues indicates impairment.
Conditions That Affect Results
The SFST battery was developed and validated in controlled laboratory settings with level floors, good lighting, and calm environments. Roadside conditions rarely match that standard, and several common factors can produce false clues that mimic impairment.
Road surfaces are a major issue. NHTSA’s own manual requires “a reasonably dry, hard, level, nonslippery surface” for the walking and balance tests. Most roads, however, feature a crown that slopes from the center toward the shoulders. Gravel, cracked pavement, grass shoulders, and debris all make heel-to-toe walking and one-legged balancing harder for anyone, sober or not.
Weather plays a role too. Rain makes surfaces slippery and puts water in your face. Cold temperatures stiffen muscles and cause shivering that can look like impaired coordination. Wind affects balance during the One Leg Stand and can make eyes water, interfering with the HGN assessment. Flashing police lights can actually trigger the eye movements officers look for during the HGN test, creating false positives from the testing environment itself.
Footwear matters more than most people realize. NHTSA acknowledges that heels higher than two inches significantly impair test performance, and officers are supposed to offer you the chance to remove them. Flip-flops can slip off during walking tests, and thick-soled work boots make balancing difficult.
Medical Conditions That Mimic Impairment
Several medical and neurological conditions produce nystagmus completely unrelated to alcohol. Inner ear disorders like benign positional vertigo, labyrinthitis, and Meniere’s disease are among the most common. Head injuries, stroke, multiple sclerosis, brain tumors, seizure disorders, and infections like Lyme disease can all cause involuntary eye jerking. Even a vitamin B12 or thiamine deficiency can produce nystagmus.
For the balance and walking tests, the list of interfering conditions is even longer. Leg, back, or inner ear problems, obesity, age over 65, and various neurological conditions can all produce clues that have nothing to do with drinking. Officers are trained to ask about medical conditions and physical limitations before administering the tests, but that screening is only as thorough as the individual officer makes it.
Whether You Can Refuse
Field sobriety tests and chemical tests (breath, blood, or urine) are legally distinct. In most U.S. states, field sobriety tests are voluntary. You generally have the right to decline them without automatic legal penalties like license suspension. Chemical tests, on the other hand, typically fall under implied consent laws. Refusing a breath or blood test after arrest can trigger automatic license revocation and other consequences, even without a conviction. In Minnesota, for example, refusing a chemical test results in license revocation and the refusal is reported to prosecutors.
The distinction matters because field sobriety tests primarily serve as tools to help the officer establish probable cause for an arrest. They are a decision aid, not a pass-or-fail exam. An officer who already suspects impairment based on driving behavior, the smell of alcohol, or your responses to questions may still arrest you regardless of whether you take or decline the SFST.