The standardized field sobriety test (SFST) is a battery of three tests used by police officers in the United States to determine whether a driver is impaired by alcohol or drugs. It provides a fairly consistent method for law enforcement to assess impairment.
The three components of the SFST are the horizontal gaze nystagmus (HGN), the walk-and-turn (WAT) and the one-leg stand (OLS). Each of these tests is used to determine specific factors that tend to manifest in specific ways when a driver is impaired.
SFST components
The HGN test involves an officer observing the eyes of a suspect as they follow a moving object, such as a pen or small flashlight, horizontally with their eyes. The officer looks for signs of nystagmus, an involuntary jerking of the eyeball, which can be exaggerated by alcohol consumption.
The WAT test assesses a suspect’s ability to complete tasks requiring attention, balance, and coordination. The suspect is asked to take nine steps, heel-to-toe, along a straight line, then turn on one foot and return in the same manner.
The OLS test requires the suspect to stand on one foot, with the other foot approximately six inches off the ground, and count aloud until told to put their foot down. These tests are designed to evaluate balance, coordination, and the ability to follow instructions, all of which can be impaired by alcohol or drugs.
Admissibility in court
The admissibility of SFST results in court typically depends on whether the tests are administered in accordance with NHTSA guidelines. When tests are administered in accordance with established guidelines, their results are considered scientifically reliable and can be presented as evidence of impairment.
Defense strategy considerations
The defense may challenge the validity of the SFST results by questioning the officer’s training, the conditions under which the tests were administered or the suspect’s conditions that could affect their performance on the tests.
In cases where SFST results are a key piece of evidence, a thorough understanding of the tests, their limitations and the conditions under which they were administered is crucial for both the prosecution and defense, as this knowledge can affect legal strategy and outcomes alike.