If you are stopped by an officer for a traffic violation and the officer suspects you may be drinking and driving, it is likely that field sobriety tests will be administered. Field sobriety tests are used as evaluations made by the officer as to whether you may be impaired.
The three most common field sobriety tests are the Horizontal Gaze Nystagmus (HGN), the Walk-and-Turn (WAT) and the One-Leg Stand (OLS). These tests were developed through research sponsored by the National Highway Traffic Safety Administration (NHTSA). A formal training program, known as DWI Detection and Standardized Field Sobriety Testing,was developed by NHTSA to train law enforcement officers as to how to better evaluate OVI or DUI suspects.
Horizontal Gaze Nystagmus is an involuntary jerking of the eye that occurs naturally as the eyes gaze to the side. When a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person may also have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight, horizontally with his or her eyes.
The Walk-and-Turn test and One-Leg Stand test are easily performed by most unimpaired people. They require a suspect to listen to and follow instructions while performing simple physical movements. In the Walk-and-Turn test, the subject is directed to take nine steps, heel-to-toe, along a straight line. After taking the steps, the suspect must turn on one foot and return in the same manner in the opposite direction.
In the One-Leg Stand test, the suspect is instructed to stand with one foot approximately six inches off the ground and count aloud by thousands. The officer times the subject for 30 seconds. The officer looks for four indicators of impairment, including swaying while balancing, using arms to balance, hopping to maintain balance, and putting the foot down.
Some information on this page was obtained from NHTSA at
http://www.nhtsa.dot.gov/People/injury/alcohol/SFST/appendix_a.htm
