The Horizontal Gaze Nystagmus (HGN) test is a field sobriety test (FST) used by Rhode Island law enforcement officers to help determine whether a driver is impaired by alcohol or drugs. This test is often used with other FSTs to gather evidence of impaired driving.
Nystagmus is an involuntary jerking of the eyes, which can be caused by various factors, including alcohol and certain drugs. The HGN test measures nystagmus in response to the movement of an object, typically a pen or finger, as it is moved horizontally across the field of vision.
During the test, the officer will ask the driver to follow the object with their eyes, without moving their head. The officer will look for three specific signs of nystagmus, known as “clues.”
These clues include a lack of smooth pursuit where the driver’s eyes do not smoothly follow the object, distinct and sustained nystagmus at maximum deviation and the onset of nystagmus before 45 degrees (The eye jerking begins before the object reaches a 45-degree angle).
Each eye is tested separately, and the officer will look for six clues (three in each eye). If four or more clues are present, it is believed that the driver’s blood alcohol concentration (BAC) is above the legal limit.
Limitations of the HGN test
While the HGN test can be a useful tool for law enforcement officers in assessing impairment, its effectiveness has some limitations. The test only detects impairment from alcohol and certain drugs, such as barbiturates and benzodiazepines. It does not detect impairment from other drugs, such as marijuana or cocaine.
Furthermore, some medical conditions, such as vertigo or nystagmus, can cause the same eye jerking that the HGN test is designed to detect. This means that a driver who is not impaired may still test positive for the HGN clues and be suspected of a DUI. Additionally, the accuracy of the HGN test is dependent on the skill of the officer administering the test. The test results may be unreliable if the officer does not follow proper testing procedures.
The HGN test is not foolproof
The HGN test is a commonly used field sobriety test that can be useful in detecting impairment from alcohol and certain drugs. However, it is important to remember that the test is not foolproof and has its limitations. If you have been charged with a DUI or DWI based on the results of an HGN test, you may want to contest the result, a process that starts with understanding your legal options.