Understanding Standardized Field Sobriety Tests

In an effort to explain how police and prosecutors attempt to prove DUI cases, it is important to understand the value of and how field sobriety tests are used in drunk driving prosecutions.

Police use a battery of three tests established and approved by the National Highway Traffic Safety Administration (NHTSA) in their effort to detect and convict drunk drivers. Prosecutors use field sobriety tests to bolster and prove driving-under-the-influence cases. The NHTSA is a federal agency, part of the Department of Transportation, whose goal is to reduce traffic accidents and related injuries and fatalities.

Through laboratory research, the NHTSA concluded that the three tests, when administered in a standardized fashion, provide a reliable indication of a person’s breath alcohol concentration (BAC) being in excess of Washington state’s legal limit of .08%. The three standardized field sobriety tests (SFSTs) authorized by the NHTSA are the horizontal gaze nystagmus (HGN), walk and turn (W&T), and one leg stand (OLS). Of those three tests, the NHTSA believes that the HGN is the most reliable field sobriety test for determining intoxication.

Horizontal Gaze Nystagmus is a medical term referring to the involuntary jerking/oscillation of the eye as the eyes move to the side. It can become more pronounced when a person consumes alcohol or other central nervous system depressants. It also can occur naturally in some individuals.

Administration of the Horizontal Gaze Nystagmus Test (HGN)

Moriarty & Associates on Standardized Field Sobriety TestsWhen conducting the HGN test, the officer will instruct the person to stand with their feet together, hands at their sides, and to keep their head still. Prior to administering the test, the officer should observe the person’s eyes to look for resting nystagmus, unequal pupil size, and/or lack of equal tracking (i.e., whether both eyes follow an object together). If any of these observations are made, the person may suffer from a medical condition that could render the test unreliable.

The officer will hold a stimulus (usually a pen or his fingertip) approximately 12–15 inches from the person’s nose and slightly above eye level. The officer then slowly moves the stimulus from one side to the other, starting to the right. The person is instructed to follow the object with his/her eyes while keeping his/her head still. The officer will look for three different clues in each eye (for a total of six) during the test.

  1. Lack of Smooth Pursuit. Starting first with the left eye, the officer will determine whether person’s the eyes jerk while following the stimulus.
  2. Distinct Nystagmus at Maximum Deviation. The next clue the officer will look for is distinct and sustained nystagmus at maximum deviation. In this portion of the test, the officer will again start with the left eye and hold the stimulus at maximum deviation (i.e., all the way out to the side) for a minimum of four seconds to determine if the eyes continue to jerk to the side.
  3. Onset of Nystagmus Prior to 45 Degrees. Finally, again starting with the left eye, the officer will look for the onset of nystagmus prior to a 45-degree angle from the person’s nose. The officer, based on his training and experience, estimates the 45-degree angle, but most officers testify that they use the person’s shoulder as a measure.

If the officer observes four or more clues, the person is considered to have failed the test and may be arrested for driving under the influence. Subsequent posts will discuss the remaining two standardized field sobriety tests and how the prosecution uses the tests to try and prove their case.

If you have been arrested for or charged with DUI, call Moriarty & Associates, PLLC, at (425) 670-0800 or fill out our free case consultation.


Attorney Patrick M. Moriarty earned his Juris Doctorate from Western New England College School of Law in 1990. He is the founder of Moriarty & Associates, PLLC, and serves as a judge pro tempore in district and municipal courts in Snohomish County. Learn more.

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