What is the primary purpose of the HGN test in DUI investigations?

The primary purpose of the Horizontal Gaze Nystagmus (HGN) test in DUI investigations is to assess eye movements as an indicator of impairment. This test is based on scientific observations that show the presence of nystagmus, which is an involuntary jerking motion of the eyes, becomes more pronounced when an individual is under the influence of alcohol or certain drugs.

During the HGN test, law enforcement officers look for specific characteristics, such as the angle at which the jerkiness occurs, which can indicate the level of impairment. The presence of nystagmus is a reliable sign that the person's ability to drive safely may be compromised due to the effects of alcohol or other intoxicants. This method does not focus on other factors such as sleepiness, driving skills, or confirming alcohol consumption directly; rather, it provides objective evidence of impairment that is critical for supporting DUI charges.