Field Sobriety Tests For Drunk Driving | Overview & Legal Considerations

When a police officer suspects someone of driving under the influence, they will likely perform a field sobriety test to assess an individual’s balance, coordination, and attention.

Three Field Sobriety Tests

There are three Standardized Field Sobriety Tests (SFST or FST) endorsed by the National Highway Traffic Safety Administration (NHTSA) in their published instructor guides. These tests are:

Horizontal Gaze Nystagmus Test (HGN)

For the HGN test, an officer will have the individual suspected of being under the influence following a moving object (often a pen, light, or finger) from side to side without moving their head or body. 

The officer then tracks the behavior of the eye very closely, noting any involuntary jerking of the eye (nystagmus), the angle at which it occurs, and the overall smoothness of the motion of the eye.

If the police officer detects any four cues between the two eyes, they will likely consider the test failed.

Walk-And-Turn Test

For this test, an officer will ask a driver to take nine heel-to-toe steps in a straight line, centering the weight of the body. After nine steps, the driver is asked to turn on one foot and come back along the same imaginary line.

The officer, meanwhile, checks for indicators of impairment. This includes failing to:

• wait to begin until instructed to do so
• keep balance while listening to instruction
• touch heel-to-toe
• continue walking without stopping
• keep balance while turning
• avoid taking an incorrect number of steps
• balance without outstretched arms

The driver fails if they violate any two of these seven criteria.

The One-Leg Stand Test/Divided Attention Test

For the final test, the officer asks the driver to stand on one foot with the other foot held around six inches from the ground. They must then count up from 1,001 until the officer tells them to put their foot down.

For the next half-minute or so, the officer will likely check to see that the driver:

• does not hop to keep balance
• does not lower their foot to the ground
• is not swaying to keep balance
• is able to balance without outstretched arms

If the driver violates any two of these criteria, they likely fail the test.

How Accurate Are FSTs For DUI/DWI Detection?

In the United States, it is illegal to operate a vehicle with a blood alcohol content (BAC) of .08% or higher. According to the NHTSA:

• if a driver fails the Horizontal Gaze Nystagmus Test, there is a 77% chance that they have a blood-alcohol content (BAC) of 0.10 or greater
• there is a 68% chance if a driver fails the Walk-And-Turn Test 
• there is a 65% chance if a driver fails the One-Leg Stand Test

Overall, SFSTs have been demonstrated to clear 90% of suspected drunk drivers who are not driving over the legal BAC, provided that the tests are performed by a properly trained officer.

Why Sober People Can Fail A Test

However, one in four people or more who fail a specific SFST will not be over the legal BAC. In fact, there are a variety of reasons why an individual who has not consumed any alcohol or other mind-altering substance may fail a SFST.

These include:

• physical disabilities
• medical conditions
• age
• simple poor balance or coordination
• fear or anxiety
• officer error

What Happens If You Fail A Field Sobriety Test?

If you fail field sobriety testing the officer will likely ask you to use a preliminary alcohol screening (PAS) device, otherwise known as a breathalyzer test.

As you blow into the device it will estimate your blood alcohol concentration (BAC) via infrared spectroscopic analysis of the vapor. 

Based on your breath test results, law enforcement officers will either arrest you, release you, or take some other action depending on the circumstances of the situation.

Are You Required To Submit To Field Sobriety Tests?

No. Individuals are not legally compelled to participate in traffic stop sobriety tests and may refuse to do so if they wish. However, officers may still perform a DUI arrest if they feel they have probable cause.

Are You Required To Submit To BAC Tests?

Yes. Refusing BAC chemical tests, including breath tests, can have serious consequences, which may vary depending on your state or local area.

Under implied consent laws, US drivers consent to alcohol testing (specifically breathalyzer testing, not blood testing in the absence of a warrant) when they receive their driver’s license. 

Refusing BAC testing can result in fines, jail time, revoking of your license, and potentially blood draws against your will if the police officers are able to obtain an electronic warrant.

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