Field sobriety tests are simple physical and cognitive exercises that are given to drivers who are thought to be under the influence of alcohol and/or drugs. During the test, the driver must perform tasks such as walking in a straight line or standing with one leg off of the ground. The law enforcement agent must grade the test to determine whether or not a drunk-driving arrest may be made.
In 1975, the National Highway Traffic Safety Administration (NHTSA) commissioned the Southern California Research Institute to study and evaluate the field sobriety tests that were most commonly used by law enforcement agencies around the country. The NHTSA wanted a set of proven field sobriety tests that could be standardized, and thus more reliable.
There were originally 16 tests that were considered. The authors had to make sure that the tests were alcohol sensitive and would reflect if a subject’s blood alcohol content (BAC) was over 0.10 percent – not merely that the driver was impaired. In the end, the study’s authors recommended the walk-and-turn test, the one-leg stand test and the Horizontal Gaze Nystagmus test.
The standardized field sobriety tests were not without problems—in fact, there was a 47 percent error margin. The authors determined that the margin was so high because many officers either did not score the tests properly or they ignored the evidence that showed some test impairment was due to factors other than alcohol.
In order to counteract the physical issues that could lead to a failing grade, the authors recommended that tests such as the walk-and-turn and one-leg stand be performed on a dry, level, non-slippery surface in safe conditions. The authors also noted that certain people, such as those over the age of 65, those with back, leg or middle ear problems, and those who are more than 50 pounds overweight, would have difficulty performing the field sobriety tests.
In 1981, a study reported that the Horizontal Gaze Nystagmus could identify an individual with a 0.10 BAC 77 percent of the time, the walk-and-turn test was correct 68 percent of the time and the one-leg stand test was correct 65 percent of the time. If the Horizontal Gaze Nystagmus test and walk-and-turn test were combined, this was determined to be effective 80 percent of the time. The authors noted that there was a 32 percent false arrest rate.
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