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Field Sobriety Test

The Standardized Field Sobriety Test was developed in consultation with scientists in the 1970s, and became common practice across the country in 1981. It is typically performed during a traffic stop, and includes 3 separate tests designed to determine whether, or to what extent, a driver is impaired:

1. The horizontal gaze nystagmus (HGN)

According to the National Highway Traffic Safety Administration, “Nystagmus is a term that describes a ‘bouncing’ eye motion that is seen in two ways — pendular nystagmus, where the eye wavers equally in two directions (like a pendulum), and jerk nystagmus, where the eye moves slowly away from a fixation point and then is rapidly corrected through a fast movement.

Horizontal gaze nystagmus is a type of jerk nystagmus and is an involuntary motion, meaning that the person showing it cannot control it, and in fact, is unaware that it is happening. When impaired, a person’s nervous system displays a breakdown in the smooth and accurate control of eye movements, resulting in a number of observable changes.” 1

During an HGN test, an examiner (typically the police officer) moves an object such as a pen or a flashlight from side to side in front of the suspect’s face in order to observe indicators of impaired function: the eye cannot follow the object, or jerks while attempting to do so, or the angle at which jerking ensues is less than 45 degrees of center. These indicators suggest impairment by alcohol, prescription medications, or illicit drugs.

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2. The walk-and-turn

The walk-and- turn test requires a suspect to take nine steps forward while setting the heel of one foot against the toes of the other foot. After nine steps, the suspect is directed to turn on a single foot and walk back to the starting point in the same manner. If the suspect cannot absorb the instructions while standing upright, or begins the test before the instructions are completed, or physically falters (e.g., by using his/her arms to maintain balance), turns improperly, or takes the wrong number of steps, this may indicate impairment and an elevated BAC.

3. The one-leg stand

In the one-leg test, a suspect is asked to stand on one foot while holding the other foot six inches off the ground, and count aloud starting at one thousand. This continues for approximately 30 seconds, or until the officer instructs the suspect to put his/her suspended foot back down on the ground. Signs of impairment include swaying, waving arms to maintain balance, and putting the suspended foot on the ground before being instructed to do so.

As noted by the National Highway Traffic Safety Administration, “The Walk and Turn and One-Leg Stand tests require a person to listen to and follow instructions while performing simple physical movements. Since these tests are alcohol sensitive, impaired persons have difficulty with these divided attention tasks. During the tests officers observe and record clues which are indicators of impairment.” 2

The findings of the Standardized Field Sobriety Tests are admissible as evidence in courts in most states. While the walk-and- turn and the one-leg test tend to be accepted without contention, the HGN is frequently challenged by defense attorneys. Nevertheless, an extensive scientific review commission by the National Highway Traffic Safety Administration concluded that the HGN is a “robust phenomenon.” 3

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