Field Sobriety Testing
The National Highway Traffic Safety Administration (NHTSA) has developed a Standardized Field Sobriety Test (SFST) in response to an increasing number of alcohol-related fatalities in the 1950s and 1960s. Beginning in 1981, police officers began using a series of roadside tests recommended by the NHTSA. These tests include: Horizontal Gaze Nystagmus (HGN), Walk-and-Turn (WAT), and One-Leg Stand (OLS).
HGN Testing
Horizontal Gaze Nystagmus is an involuntary jerking of the eye that occurs naturally as the eyes gaze to the side. Under normal circumstances, nystagmus occurs when the eyes are rotated at high peripheral angles. However, when a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person will also often 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 examiner looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center. If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.08 or greater. NHTSA research found that this test allows proper classification of approximately 88 percent of suspects (Stuster and Burns, 1998). HGN may also indicate consumption of seizure medications, phencyclidine, a variety of inhalants, barbiturates, and other depressants.
Walk and Turn
The Walk-and-Turn test and One-Leg Stand test are “divided attention” tests that are easily performed by most unimpaired people. They require a suspect to listen to and follow instructions while performing simple physical movements. Impaired persons have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises.
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. The examiner looks for eight indicators of impairment: if the suspect cannot keep balance while listening to the instructions, begins before the instructions are finished, stops while walking to regain balance, does not touch heel-to-toe, steps off the line, uses arms to balance, makes an improper turn, or takes an incorrect number of steps. NHTSA research indicates that 79 percent of individuals who exhibit two or more indicators in the performance of the test will have a BAC of 0.08 or greater.
