If a New Hampshire police officer makes a traffic stop, the driver may be asked to exit and submit to a preliminary alcohol screening. While witnessing a swerving vehicle is a legitimate reason for making a traffic stop, it does not typically constitute grounds for an arrest. Before administering any field sobriety testing including an eye test, the officer must first establish probable cause for a suspected DUI.
The horizontal gaze nystagmus (HGN) test is an eye test that police officers use to determine whether a person may have been intoxicated while driving. As with all field sobriety tests, a driver is not obligated to comply with a request to take the test. If he or she agrees to do so, however, the patrol officer will be closely monitoring eye movement to see if nystagmus occurs, which is an erratic jerking of the eyeball.
Everyone has a maximum peripheral vantage point where nystagmus occurs
During an HGN test, a police officer typically holds up a pen or a finger and instructs the participant to track the object left to right or up and down without moving his or her head. Everyone has a maximum peripheral vantage point where an eyeball will begin to jerk erratically while tracking an object. When a person is intoxicated, this eyeball movement will occur much sooner than it might in a sober person.
Failing an HGN test (which, by the way, is determined at the discretion of the police officer) constitutes probable cause to make a DUI arrest. Some people have vision problems or eye conditions that may impede their ability to perform well on these tests. Anyone who is facing DUI charges in New Hampshire following an HGN or other field sobriety test will likely benefit by consulting with an attorney before heading to court, especially if he or she believes the search and seizure process violated Fourth Amendment rights.