A lot can ride on this test: arrest decisions, license problems, higher defense costs, and pressure to plead guilty based on what many people assume is a foolproof sign of intoxication. It is not. Horizontal gaze nystagmus is an involuntary jerking of the eyes that can become more noticeable as someone looks to the side. During a roadside exam, an officer moves a stimulus, such as a penlight, across the driver's field of vision and watches for clues like lack of smooth pursuit, distinct jerking at maximum deviation, and onset of jerking before about 45 degrees.

The catch is that this is only one field sobriety test, not a chemical measurement like a breath test or blood test. Fatigue, certain medications, neurological conditions, eye issues, flashing lights, head injuries, and harsh roadside conditions can affect performance. On dark, icy highways patrolled by the Alaska State Troopers, those factors are not theoretical.

In a DUI case, the result may be used to support probable cause for arrest and later as part of the prosecution's evidence. But bad advice says failing it automatically proves impairment. It does not. How the officer administered the test, the person's medical history, and whether the observations were documented correctly can all shape a defense. If the same stop led to a crash injury claim, Alaska's 2-year statute of limitations for personal injury cases can also matter.