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Field Sobriety Tests
 
 
Field Test During the years 1975 - 1981, a battery of field sobriety tests was developed under funding by the National Highway Traffic Safety Administration (NHTSA), U.S. Department of Transportation (Bums and Moskowitz, 1977; Tharp Burns, and Moskowitz, 1981). The tests include Walk-and-Turn (WAT), One-Leg Stand (OLS), and Horizontal Gaze Nystagmus (HGN). NHTSA subsequently developed a training curriculum for the three-test battery , and initiated training programs nationwide.

Traffic officers in all 50 states now have been trained to administer the Standardized Field Sobriety Tests (SFST's) to individuals suspected of impaired driving and to score their performance on the tests. At the time the SFST's were developed , the statutory blood alcohol concentration (BAC) for driving was 0.10% throughout the United States. The limit now has been lowered in a number of states to 0.08% for the general driving population. "Zero tolerance" is in effect in some jurisdictions for drivers under age 21, and commercial drivers risk losing their licenses at a BAC of 0.04%.

The standardized battery includes three tests: Horizontal Gaze Nystagmus, Walk and Turn, and One Leg Stand.

Horizontal Gaze Nystagmus (HGN)
To conduct an HGN examination, an officer instructs the driver to hold his head still and to follow the movement of a stimulus with his eyes. The examination for three signs in each eye yields a possible total score of six. The three signs are:
1 Lack of smooth pursuit: The eyes do not smoothly pursue the moving stimulus but instead move in a jerky manner.
2. Nystagmus at maximum deviation: A distinct jerking occurs when the eyes are deviated as far as possible in the horizontal plane.
3. Onset of the jerking movement prior to 45 degrees: Nystagmus occurs and persists at an angle of gaze less than 45 degrees.

* HGN occurs not only in the presence of alcohol, but also in the presence of other impairing substances (i.e. depressants, inhalants, and phencyclidine).

Walk-and-Turn Test (WAT)
A suspect is given detailed, standardized instructions for walking nine heel-to-toe steps along a line, turning, and returning along the line with nine heel-to-toe steps. A final score reflects an individual's ability to attend to and remember very specific instructions as well as to balance and walk. Officers record errors with the following checklist.

1. Loses balance during instructions
2. Starts before instructions are finished
3. Stops while walking
4. Doesn't touch heel-to-toe
5. Steps off line
6. Uses arms for balance
7. Loses balance while turning/incorrect turn
8. Can't perform test

One-Leg Stand (OLS)
This balance test requires the individual to stand with his arms at his side and to lift one leg approximately six inches off the ground. He is instructed to look at the raised foot and to maintain the stance while counting one thousand-one, one thousand-two, one thousand-three, etc." until told to stop.
The test is timed for 30 seconds and scored as shown below.

1. Sways while balancing
2. Uses arms for balance
3. Hopping
4. Puts foot down
5. Can't perform test

* variables other than alcohol can affect balance (i.e. knee, ankle, or back injuries)
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