The horizontal gaze nystagmus test (HGN), is a simple test used by the police to identify if a person is intoxicated or impaired when driving. The HGN looks at the eye movements and determines if the eyes are jerking when looking from side to side. Alcohol and some drugs make the jerking motion of the eyes more obvious, suggesting impairment.
This test isn't foolproof. There are medical conditions and medications that can cause the eyes to jerk and result in the failure of the test. This is just one of several field sobriety tests that you may be asked to take that doesn't truly represent your level of intoxication or impairment.
When all three field sobriety tests are used, they are still only 83 percent accurate, according to the National Highway Traffic Safety Administration (NHTSA). That means that 17 percent of people who fail these tests aren't intoxicated or impaired at all. The HGN is the most accurate test of the field sobriety tests, but it is not 100 percent accurate.
The officer looks for three things when you take this test. First, he or she looks for your eye to move smoothly from one side to the other. Next, he or she looks for jerking motions if you look one way for four seconds. Finally, he or she looks to see if your eye jerks when an object is 45 degrees from the middle of your face.
This cannot be the only tool used to make your arrest. It must be combined with other field tests and a Breathalyzer test at the very least. Our site has more about what to expect if you're pulled over and have your BAC tested.