Field Sobriety Tests (FSTs): An Informative Guide and Your Rights

An overview of the field sobriety tests police officers use to check for driving under the influence

By Canaan Suitt, J.D. | Last updated on July 31, 2023 Featuring practical insights from contributing attorney Mark Thiessen

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Every state has strict laws against drunk driving. To enforce state DUI/DWI laws, police officers are trained to administer tests to determine if a driver is impaired by alcohol or drugs.

If a law enforcement officer sees a driver committing traffic violations, such as speeding or swerving between lanes, they can pull the driver over. If the officer then sees signs of intoxication, they can ask the driver to take a Standard Field Sobriety Test (SFST).

If the driver takes and fails the SFST, that gives the police officer evidence of impairment and probable cause to arrest the driver on DUI/DWI charges.

This article gives an overview of the three tests that comprise the SFST, including:

  • How each test works;
  • What it takes to fail the tests;
  • And what happens if you fail;
  • Whether you can fight test results in court.

If you failed an SFST and have been charged with a DUI or DWI, the wisest course of action is to consult with an experienced DUI defense attorney as soon as possible.

What Are Field Sobriety Tests (FSTs)?

Field sobriety tests are standardized tests that law enforcement officers are trained to administer to determine if a driver they suspect of impairment is actually under the influence of alcohol or drugs.

“A police officer needs a reasonable suspicion to pull you over. Then, once they stop and detain you, they need reasonable suspicion to continue the detention beyond just the traffic stop,” says Mark Thiessen, a criminal defense lawyer in Houston specializing in DUI/DWI law.

Traffic violations such as speeding or swerving can give an officer the reasonable suspicion they need to pull you over (learn more about reasonable suspicion and traffic stops).

At that point, “If a police officer sees signs of intoxication—or signs of behavior that may indicate intoxication—then they’re allowed to prolong their investigation and detention.”

Thiessen says that common signs of intoxication include:

  • The odor of alcohol
  • There are open containers of alcohol in the car
  • You admit to coming from a bar
  • It’s 2 am, and you’re in a bar-heavy area
  • You have slurred speech
  • Your eyes are red and glassy
  • You appear disheveled
  • You’re covered in vomit

“If a law enforcement offer sees these sorts of indicators of intoxication, they’ll say, ‘In order to measure whether we think you’re intoxicated or not, we’re going to give you some tests that we’ve all been trained to administer. We’d like you to perform the standard field sobriety test.’”

You do a test you’ve never even heard of before, and you only get the instructions once… and they don’t even tell you everything they’re looking for… We just educate the jury on how meticulous these tests are and how easy they are to fail. And once juries see how meticulous these tests are, they’re like, ‘This is ridiculous.’

Mark Thiessen

What Are the Three Types of Field Sobriety Tests?

The Standard Field Sobriety Test (SFST) consists of three separate standardized tests:

  • Horizontal gaze nystagmus test (HGN test);
  • Walk and turn test (WAT test); and
  • One leg stand test (OLS test).

Each test has specific procedures and “clues” or signs of impairment that police officers are trained to look for.

1. The Horizontal Gaze Nystagmus Test (HGN)

The term “nystagmus” means the involuntary jerking of the eye.

The HGN test is “the eye test where police officers are going to see if there’s any involuntary jerking of the eyes as the person gazes to the side,” explains Thiessen.

The basic idea behind the test is that eye movement is less controlled and becomes more exaggerated under the influence of alcohol or drugs.

How Is the HGN Test Administered?

To conduct the HGN test, a police officer first prepares the driver by positioning them away from flashing lights and ensuring they don’t have a medical condition or are wearing contacts that would skew the results.

The officer places an object or “stimulus” (often the tip of a pen or penlight) approximately 12-15 inches from the driver’s face and moves it from side to side.

The officer asks the driver to follow the object’s movement with their eyes. The police officer is looking to see if the driver can track the movement smoothly or if there are any sudden jerks as the object moves to their outer area of vision.

What Are the Clues for the HGN?

According to the SFST training handbook developed under the National Highway Traffic Safety Administration (NHTSA), there are three clues for the HGN test. Since each eye is tested separately, there is a total of six clues that police officers can mark in their report:

  1. Lack of smooth pursuit of the stimulus with the eyes;
  2. Sustained nystagmus at the farthest point the person can gaze;
  3. The onset of nystagmus prior to a 45-degree angle of gaze.
How Many Clues Do You Need to Fail the HGN?

Four out of a possible six clues are enough to fail the HGN.

A DUI Defense Lawyer’s Critique of the Test’s Validity

“The HGN test is given by police officers with no medical training,” says Thiessen. “They were never trained by doctors; they were trained by fellow police officers. And they’re looking for what they think are millimeters of jerking of the eye. And they never say how many clues they see—they just write down that they saw signs ‘indicating impairment’ afterward. There are so many false positives on the HGN test.”

2. The Walk and Turn Test (WAT)

After the HGN test, “You take the nine-step walk and turn,” says Thiessen.

The WAT is a divided attention test, which tests the person’s coordination and ability to perform multiple tasks simultaneously. The idea is that alcohol impairment makes it difficult to multi-task, which sober individuals could easily accomplish.

How Is the WAT Test Administered?

The WAT is simple in theory. The police officer first gives the instructions; then, the subject performs the test.

On an imaginary line, you take nine heel-to-toe steps. At the end of the line, you turn on one foot and walk back the same way on the straight line.

What Are the Clues for the WAT?

There are eight clues that police officers look for during the WAT test:

  1. The person can’t maintain balance while getting test instructions;
  2. They start walking too soon;
  3. They stop walking;
  4. They can’t touch their heel to their toe;
  5. They step off the line;
  6. They have to use their arms to balance themselves;
  7. They take the incorrect number of steps;
  8. They make an improper turn at the end of the line.
How Many Clues Do You Need to Fail the WAT?

Two out of a possible eight clues are enough to fail the WAT.

A DUI Defense Lawyer’s Critique of the WAT Test’s Validity

The WAT test is problematic because “the police officers only give you the instructions one time, demonstrate it once, and only need two clues to indicate impairment. That means that in order to pass it, you have to get zero clues or one clue. And it’s a very meticulous test,” says Thiessen.

“These tests are highly subjective, and honestly, they’re designed for failure.”

3. One-Leg Stand Test (OLS)

The third test in the SFST is called the one-leg stand. Like the WAT, the OLS is a divided attention test, requiring the person to focus on multiple tasks at the same time. The idea is that individuals who are impaired by alcohol cannot successfully perform multiple tasks simultaneously.

How the OLS Test is Administered

The OLS is simple. “On the one-leg stand, you stand and hold your leg six inches off the ground for 30 seconds,” says Thiessen.

What Are the Clues for OLS?

There are four possible clues or indicators of impairment in the OLS:

  • The person sways while balancing;
  • They use their arms to balance;
  • They hop around;
  • They put their foot down.
How Many Clues Do You Need to Fail the OLS?

Two out of a possible four clues are enough to fail the OLS.

A DUI Defense Lawyer’s Critique of the OLS Test’s Validity

“There is no definition of how far you have to sway in the OLS,” says Thiessen. “So, while you’ve got your arms at your side, your leg straight and lifted six inches, and you’re looking down at the ground—if, during all of this, the police officer sees any noticeable sway, that’s already one clue. Then, if you drop your foot before 30 seconds is up or you hop, you’re already indicating impairment.

“This test is easy to fail,” continues Thiessen, and it ignores the fact that a person may simply be nervous, uncoordinated, or inexperienced in dealing with law enforcement.

Can You Refuse a Field Sobriety Test? 

Field sobriety tests are generally considered voluntary under state laws. So, you can politely refuse to take an FST without legal consequences.

However, refusing to take an FST doesn’t mean the end of a traffic stop. With reasonable suspicion of intoxication, a police officer can then ask you to take a breath test to measure your blood alcohol content (BAC).

You can also refuse to take a breath test, but under state implied consent laws, there will be legal consequences for refusal, such as your driver’s license suspension. In some states, your refusal to submit to a breathalyzer is admissible in court.

What Happens If I Fail the Field Sobriety Tests?

If you fail the FSTs, a police officer will have probable cause for a DUI arrest. They may ask you to complete a breath test at the roadside by breathing into a portable breathalyzer. Or they will immediately proceed with the DUI arrest and take you to the police station.

At the station, the officer will book you with DUI charges and administer further blood alcohol concentration (BAC) testing, such as an additional breath test or a chemical test using a blood or urine sample. Blood tests can also be used to test for the presence of drugs in addition to alcohol.

Learn more about BAC levels and breath tests.

Can You Challenge Field Sobriety Test Results?

The bad news is that “by the time you’re doing the standard field sobriety test, you don’t stand a chance,” says Thiessen.

“You do a test you’ve never even heard of before, and you only get the instructions once. You only get the demo once, and they don’t even tell you everything they’re looking for. And, of course, the police officer is going to say they saw these millimeters of eye jerking on the HGN. It’s all so easy to fail.”

The good news is that one major line of defense in DUI cases is to challenge the improper administration or validity of FSTs.

“Not even challenge—we just educate the jury on how meticulous these tests are and how easy they are to fail,” Thiessen adds. “And once juries see how meticulous these tests are, they’re like, ‘This is ridiculous.’ They all go back and try doing the tests themselves during deliberation and think, ‘I’m sober right now, and I’m failing this.’”

Getting a DUI Defense Attorney to Protect Your Legal Rights

Criminal defense lawyers with experience in DUI cases will understand your state’s DUI laws and legal defenses. They’ll understand the court procedures and how best to pursue your defense strategy. Ultimately, a DUI defense attorney’s goal is to protect your legal rights throughout the legal process and ensure that all defenses are brought before the court promptly.

DUI law varies by jurisdiction and constantly changes within each state. Furthermore, the consequences of a DUI conviction are severe and life-altering, from fines and potential jail time to driver’s license suspension and negative employment impacts. It’s not something to handle on your own and without consulting a legal expert.

The good news is that many DUI/DWI lawyers provide free consultations to learn about your case.

So, if you have been arrested for driving under the influence (DUI) or driving while intoxicated (DWI), consult a criminal defense lawyer with experience in DUI cases for legal advice as soon as possible.

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