Field Sobriety Test to Assess Drunk Driving

Field Sobriety Tests are groups of three tests used by police to determine if a driver is impaired. The tasks assess balance, coordination, and the ability of the driver to divide his attention to more than one task during the field sobriety test.

The Standard Field Sobriety Test (SFST) is a battery of 3 tests that include the horizontal gaze nystagmus (HGN), the walk-and-turn, and the one-leg stand tests.

These tests have been scientifically proven to validate legal intoxication in drivers suspected of drunken driving in 90% of cases if administered by a trained officer. Results of the test are admissible as evidence in court.

National Highway Traffic Safety Administration defines and describes the three parts of the SFST in detail:

Horizontal Gaze Nystagmus

Horizontal gaze nystagmus is an involuntary "jerking" of the eyeball which happens to everyone when the eyes are rotated at high peripheral angles. When a person is intoxicated, however, the jerking of the eyes becomes more exaggerated and occurs at lesser angles.

Turning the HGN test, the officer will ask the driver to follow a moving object, such as a pen or flashlight, slowly from side to side. The officer looks to determine:

• If the eye cannot follow the object smoothly
• If jerking is distinct when the eye is at maximum deviation
• If the angle of jerking onset is within 45 degrees

If four or more clues appear between the two eyes, the driver is likely to have a blood-alcohol content (BAC) 0.10 or greater. NHTSA research shows this test to be accurate in 77% of test subjects.

Walk-And-Turn Test

For the walk-and-turn test, the officer asks the driver to take nine steps, heel-to-toe, along a straight line, turn on one foot and return nine steps in the opposite direction.

During the test, the officer looks for seven indicators of impairment:

• If the suspect cannot keep balance while listening to the instructions
• Begins before the instructions are finished
• Stops while walking to regain balance
• Does not touch heel-to-toe
• Uses arms to balance
• Loses balance while turning
• Takes an incorrect number of steps

If the driver exhibits two or more of the above indicators during the test, there is a 68% likelihood of at BAC level of 0.10 or higher, according to the NHTSA.

One-Leg Stand Test

For the one-leg stand test, the officer asks the driver to stand with one foot about six inches off the ground and count by from 1,001 (one-thousand-one, one thousand two, etc.) until the officer says to put the foot down.

During the next 30 seconds, the officer looks for these four indicators:

• Swaying while balancing
• Using arms to balance
• Hopping to maintain balance
• Putting the foot down

If the driver exhibits two or more of the above indicators, there is a 65% chance he has a BAC of 0.10 or greater, according to the NHSTA.

If the driver fails any of the above field sobriety tests, the officer will then ask the suspect to take a breath test or a chemical test to confirm their blood-alcohol content.

Other Reasons for Failing These Tests

There are many reasons that people who are not intoxicated might not be able to perform the above tests successfully, including certain medical conditions, disabilities, age, injury, and taking a certain medication. Wearing contact lenses, for example, could affect the HGN test results.​

The officer usually will ask the driver if there is a reason that they may not be able to pass the test and makes a note of their answer in his arrest report.

If there is a legitimate reason, medical or otherwise, why you might fail one or more parts of the field sobriety test make sure you mention it to so that the officer makes a note of it in the official record. It might be helpful to you in court later.