You might see this in an arrest report, a prosecutor's packet, or an officer's notes after a traffic stop: "HGN observed," "6 of 6 clues," or "lack of smooth pursuit." It means an officer says they watched a person's eyes as they followed a moving object side to side and noticed involuntary jerking. That eye movement is called nystagmus. In DUI investigations, horizontal gaze nystagmus is one of the standard field sobriety tests used to look for signs that alcohol or certain drugs may be affecting the brain and nervous system.

Practically, this matters because it can become part of the officer's claimed basis for probable cause to arrest or request a chemical test. But it is not a breath test, and it does not directly measure blood alcohol concentration. Fatigue, medical conditions, eye problems, head injury, flashing lights, and poor roadside conditions can all complicate what the officer thinks they saw.

In Hawaii, HGN often shows up in OVUII cases under Hawaii Revised Statutes § 291E-61 (2024). Officers may cite it along with driving behavior, speech, odor of alcohol, and performance on other roadside tests. On rural islands, where hospital access can be limited and crash response times may run longer, roadside observations can take on extra weight. That makes the accuracy, training, and documentation behind an HGN test worth close attention in any injury-related or criminal case.