DUI Investigation Roadside Testing: The Horizontal Gaze Nystagmus (HGN) Test
This is an overview of the NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION (NHTSA) rules for STANDARDIZED FIELD SOBRIETY TESTING (SFST) for HGN. This testing is used by law enforcement in the most important phase of a roadside DUI investigation the officer's observations of the driver.
Horizontal Gaze Nystagmus (HGN):Standardized Field Sobriety Tests consist of three tests, the HGN, Walk and Turn and One Leg Stand. These tests are designed to be given with precision and in the correct order.
Horizontal Gaze Nystagmus (HGN):
Nystagmus is a medical term that is said to describe the involuntary jerking or motion of the eyeballs. When someone is intoxicated by a central nervous system depressant such as alcohol and some drugs, this involuntary jerking becomes more pronounced. Horizontal Gaze Nystagmus is often described as similar in appearance to how a dry windshield wiper jerkily moves across a dry windshield.
In theory, the horizontal gaze nystagmus (HGN) test is used by law enforcement to check a motorist nystagmus in order to determine if probable cause exists to authorize an arrest for the offense of drunk driving (DUI). The Theory does not always have real life application and I see many cases where the HGN testing is not done correctly. Incorrect testing is not worth doing and frequently leads to misinterpretation.
HGN Test Procedure:Before conducting a HGN or horizontal gaze nystagmus test, the officer is required to evaluate your eyes to look for resting nystagmus, equal pupil size and equal tracking or movement of the eyeballs together. Without excluding these elements it is not scientifically possible to rule out an existing medical condition or injury that will inhibit accurate administration of the HGN testing.
To give the test, the police officer will hold a small object (usually a pen or a light) approximately 12 inches from your nose and slightly above eye level. The LEO will slowly move it from one side to the other. You will follow the object or stimulus with your eyes while holding your head as still as possible.
The law officer will look for three different clues in each eye,total of six clues during the test. One clue is a lack of smooth pursuit*where the eyes are jerking or bouncing while tracking the stimulus. If lack of smooth pursuit is detected, the officer will check for nystagmus the begins before the eyes reach a 45 degree angle. If, and only if, the prior clues are noted, the LEO will look for the last set of clues which is known as the onset of nystagmus prior to maximum deviation. This clue is only counted if the eyes begin jerking and continue to jerk for four seconds before the eye is looking all of the way to the side.
If the police officer believes he sees four or more clues, he has probable cause to make an arrest for DUI.
Problems with HGN in the Field:Horizontal Gaze Nystagmus testing can and should be reviewed and challenged in many cases. Even when HGN is administered in the most ideal of scenarios, the horizontal gaze nystagmus is only 77% accurate in determining if an individual is impaired. In the past 25 years of DUI practice Mo Wiltshire has seen many many cases where the HGN testing was done poorly and unreliably and we frequently raise successful challenges to the application of this testing. HGN is difficult to conduct accurately on the side of the road, at night, with traffic whizzing by and otherwise under the conditions that are present in most DUI cases.