Law enforcement officers in Texas often rely on the results of field sobriety tests to help them determine whether a motorist is impaired by alcohol or drugs. The test is standardized and consists of three exercises developed during research conducted at Southern California Research Institute. The research was funded by the National Highway Traffic Safety Administration. NHTSA also developed the training police officers receive to evaluate these tests.
The first exercise that an individual suspected of drunk driving will likely be asked to perform is called the horizontal gaze nystagmus test. During this test, the officer observes the drivers eyes as an object passes from side to side. The eyes will jerk involuntarily in all individuals when the eye rotates at sharp angles, but the eyes of an individual who has been drinking will jerk at shallower angles.
The other two exercises are referred to as divided attention tests. These tests require individuals to listen to instructions while performing simple tasks. The first test involves walking in a straight line heel-to-toe before turning and walking back to the starting point. During the second test, the individual is told to lift one foot approximately six inches off the ground while counting aloud.
The research supporting these tests indicate that they may not always be reliable. The results of the horizontal gaze nystagmus test may be misleading 23 percent of the time, and divided attention tests are only 65 percent reliable. These questions could be raised by a criminal defense attorney when field sobriety test results are included in a drunk driving case. An attorney could also point out that many people, including those who are overweight or over the age of 50, would have great difficulty passing a standardized field sobriety test even when completely sober.
Source: National Highway Traffic Safety Administration, "The Highway Safety Desk Book", November 22, 2014