Field Sobriety Tests Explained: The Three Standardized Tests

If an officer suspects you have been drinking during a traffic stop, they will often ask you to step out and perform a series of roadside exercises. These are field sobriety tests, and they are not the casual, friendly check they may appear to be. They are a structured scoring system designed to build evidence against you. Understanding how they work, what officers are looking for, and whether you even have to do them puts you in a far stronger position.

What the field sobriety test acronym (SFST) means

The full name is the Standardized Field Sobriety Test battery, usually shortened to SFST. It was developed and validated by the National Highway Traffic Safety Administration (NHTSA) in the 1970s and refined since. "Standardized" is the key word: officers are trained to give specific instructions, demonstrate each test the same way, and count specific, predefined "clues." When the test is not administered exactly as NHTSA prescribes, the results lose much of their claimed scientific value, which is one of the most common ways defense lawyers attack them.

There are only three tests in the standardized battery. Other roadside exercises you may have heard of, such as reciting the alphabet, counting backward, or touching your finger to your nose, are not part of the validated SFST and carry even less scientific weight.

The three standardized tests and their clues

1. Horizontal Gaze Nystagmus (HGN)

Nystagmus is an involuntary jerking of the eye. Alcohol and certain drugs can make this jerking more pronounced as the eyes track to the side. The officer holds a pen, fingertip, or small light about a foot from your face and moves it slowly side to side, watching your eyes.

The officer scores up to six clues, three per eye: lack of smooth pursuit, distinct and sustained nystagmus at maximum deviation, and onset of nystagmus before the eye reaches 45 degrees. Four or more clues is treated as a sign of impairment. The catch: nystagmus can be caused by dozens of innocent factors, including certain medications, neurological conditions, inner-ear problems, fatigue, and even the flashing lights of the patrol car. Most drivers have no way to know how their eyes performed, which makes HGN hard to challenge without a lawyer and an expert.

2. Walk-and-Turn

This is a "divided attention" test, meaning it asks you to follow instructions while performing a physical task. You stand heel-to-toe on a real or imaginary line, take nine steps forward, turn in a prescribed way, and take nine steps back, counting out loud and watching your feet the whole time.

The officer watches for eight clues: starting too soon, stopping while walking, not touching heel to toe, stepping off the line, using arms for balance, an improper turn, taking the wrong number of steps, and being unable to keep balance during the instructions. Two or more clues is scored as failure. Notice how low that bar is, and how many of these "clues" a perfectly sober person can produce on gravel, in a slope, in poor lighting, in bad weather, in heeled shoes, or while nervous.

3. One-Leg Stand

You raise one foot about six inches off the ground, keep both arms at your sides, and count out loud ("one thousand one, one thousand two...") until told to stop, usually about 30 seconds.

The officer looks for four clues: swaying, using arms for balance, hopping, and putting the foot down. Two or more is a fail. Balance on one leg for 30 seconds is genuinely difficult for many sober people, especially anyone over 65, anyone who is overweight, or anyone with a back, leg, knee, or inner-ear condition. NHTSA itself acknowledges these limitations.

How accurate are field sobriety tests?

Even NHTSA's own validation studies do not claim these tests are highly reliable. The often-cited figures suggest HGN is roughly 77 percent accurate, walk-and-turn about 68 percent, and one-leg stand about 65 percent at predicting a blood alcohol level at or above the legal limit, and only when administered perfectly. That means a meaningful share of sober people "fail." Age, weight, injuries, medical conditions, footwear, road surface, weather, and ordinary nervousness all generate false clues. The tests measure your ability to perform an unfamiliar exercise under stress on the roadside, not your blood alcohol content.

Do you have to take a field sobriety test?

In most states, field sobriety tests are voluntary. There is generally no criminal penalty and no automatic license suspension for politely declining the three SFST exercises. This is a crucial distinction from a post-arrest chemical test (breath or blood at the station), which is covered by your state's implied consent law and can carry an automatic license suspension if you refuse. Roadside exercises and roadside handheld breath tests are usually in a different, voluntary category, though a few states attach minor consequences and the rules vary, so it is worth knowing your own state's law in advance.

Because the tests are designed to produce evidence against you and are scored subjectively by the same officer who already suspects you, many drivers choose to decline them respectfully. Your refusal generally cannot be used as direct proof of guilt the way a chemical test refusal can, although a prosecutor may comment on it in some states.

What to say and do at the roadside

The traffic stop itself only requires reasonable suspicion, and an arrest requires probable cause. Field sobriety tests are one of the main tools officers use to build that probable cause. You can be cooperative and calm while still protecting yourself:

• Provide your license, registration, and insurance when asked. You generally must identify yourself.
• You can invoke the right to remain silent on questions like "How much have you had to drink tonight?" A simple "I'd prefer not to answer questions" is enough.
• If asked to perform field sobriety tests, you may decline politely: "Officer, I respectfully decline to do any roadside tests." Do not argue or lie.
• Stay calm, keep your hands visible, and do not physically resist even if you are arrested. Fight the case in court, not on the street.
• Remember details: lighting, road surface, weather, your footwear, and anything the officer said. These matter later.

This is general legal information, not legal advice. DUI laws, implied-consent rules, and the consequences of refusing roadside tests vary significantly by state and change over time. If you are facing a DUI charge, talk to a qualified attorney licensed in your state about your specific facts.