Once the officer notices signs of intoxication in the suspected driver, he or she will subject the suspect/driver to a battery of field sobriety tests. These tests are used to determine if the suspect driver is sober. The three most commons field sobriety tests are: the one legged stand, the Horizontal Gaze Nystagmus and the walk and turn.
One legged stand
As the name suggests, this test involves standing on one leg. The suspect/driver is asked to stand on one leg with the other leg 6 inches off the ground. The logic behind this test is that an intoxicated person will have difficulty balancing on one leg.
Horizontal Gaze Nystagmus
The Horizontal Gaze Nystagmus test involves the involuntary jerking of the eye. The officer will move a small pen like device from left to right and ask the suspect driver to look at the device as it moves from left to right and back from right to left. This test is based on the assumption that intoxication results in involuntary jerking of the eye.
Walk and Turn
In this test the officer will ask the suspect to walk along a straight line and as he or she is walking, the officer will instruct him or her to perform simple physical actions. This test is based in the logic that an intoxicated person will lack or-ordination and therefore will be in a position to perform the functions while walking.
Besides these three tests which were developed by the National Highway Traffic and Safety Administration, local police departments may also subject to suspect driver to other sobriety tests. Some common tests include reciting the English alphabets in the reverse order, touching the nose with the eyes closed, etc.
More often than not, the suspect driver will fail these tests. There are not fixed rules for determining what performance is counted as a pass and what is a fail. The officer who administers the test is the judge, jury and executioner. He decides which test you must perform and he decides whether you pass or you fail. The officer would have made the decision to arrest you even before he asked you to perform the test.