A field sobriety test, also commonly known as a roadside sobriety test is a tool used by law officers to enforce state and federal DUI laws and is normally administered before giving a Breathalyzer test. If an officer of the law suspects that a motorist is impaired either due to alcohol consumption or for some other reason, then they will usually instruct the motorist in performing a three part sobriety test.
The purpose of the sobriety tests is to allow the officer to gauge the ability of the driver to stay balanced, as well as observe their attention level and physical ability. They can then use these factors to determine if the individual is exhibiting behavior that is congruent with someone who is driving impaired. Essentially the purpose of administering a field sobriety test is to ensure that if a person is arrested and charged with a DUI offense that the arresting officer had valid probable cause.
The Standardized Field Sobriety Test is comprised of three parts. The first part is called the horizontal gaze nystagmus which tests for exaggerated involuntary jerking of the eye that occurs when the eye gazes to the side when a person is intoxicated.
The second part is called the walk-and-turn which tests the motorist’s ability to complete tasks while their attention is divided. Typically the motorist is required to take nine steps, heel-to-toe, along a straight line. Then they are instructed to turn on one foot and return to their starting position walking heel-to-toe once more.
The third and final part of the test is referred to as the one-leg stand. The motorist is instructed to lift one foot above the ground by about six inches while counting for 30 seconds. If the motorist exhibits an impaired balancing ability or is unable to complete the task without swaying, jumping or putting the raised foot down is usually taken as an indicator for possible impairment.
Source: FindLaw, “Field Sobriety Tests,” Accessed Feb. 23, 2015