Prior to the standardizing of field sobriety tests in the late 1970s and early 1980s, police officers throughout the United States used a variety of methods to determine if drivers were driving under the influence of alcohol or drugs. The National Highway Traffic Safety Administration (NHTSA), with assistance from the Southern California Research Institute, worked together to develop tests they believed were most effective in identifying intoxicated drivers.
The Standardized Field Sobriety Test includes a battery of three tests including the Horizontal Gaze Nystagmus (HGN) which measures the amount of jerking in a driver's eye as it follows a stimulus in a horizontal direction which is moved in front of the driver's face by a law enforcment officer. This natural occurrence of nystagmus is exaggerated in drivers who are intoxicated by alcohol and can be detected when the driver's eyes are rotated at lesser angles than a non-intoxicated driver.
The next test is the walk-and-turn test which requires the driver to walk heel-to-toe down a straight line for nine steps, turn on one foot and return down the same line. Drivers who are unable to follow directions, who fail to walk the required number or tests or who are unable to maintain their balance may be intoxicated.
The last test is the one-leg stand. This test requires the driver to stand with one leg raised off the ground approximately six inches and count aloud for approximately 30 seconds. Drivers who fail to count properly, who lose their balance, who lower their foot, or who cannot follow directions may be intoxicated.
According to the National Highway Traffic Safety Administration, "when the component tests of the SFST battery are combined, officers are accurate in 91 percent of cases, overall, and in 94 percent of cases if explanations for some of the false positives are accepted (Stuster and Burns, 1998)". Critics argue that the percentage of success has varied with other testing.
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