The Field sobriety tests used in DUI cases are designed to help law enforcement detect drunk drivers and objectively gauge their impairment. They accomplish this by testing a person’s motor skills, balance and coordination and ability to divide attention.
If a person is not impaired by alcohol, they should do well on these tests. If a person is impaired, they should do poorly on these tests. The main tests that are used in DUI cases are the:
Horizontal Gaze Nystagmus
Horizontal gaze nystagmus is a condition where the muscles in the eye twitch due to alcohol impairment. This twitch is call “nystagmus” and is apparent when the person stands at attention and looks with a “gaze” upon a stimulus that is moved across the horizon. While the stimulus is moved, the eye will exhibit a slight twitch (nystagmus) as the muscles in the eye twitch.
Walk the Line “Heel to Toe” Test
The “Heel To Toe” test is primarily a test of balance, but it also tests coordination, ability to listen and remember instructions(during the instruction phase) and division of attention. If someone is impaired by alcohol, they cannot properly balance.
During the test, the driver is instructed to walk along an imaginary line. Instead of walking normally, the driver is told step each step with the heel of the front foot placed directly in front of and touching the toe of the rear foot.
Stand on One Leg
Another Field Sobriety Test is the Stand on One Leg Test. The stand on one leg test primarily tests balance and a person impaired by alcohol will fail the test if they cannot stand on one leg for the prescribed 30 seconds. As with the other FST’s, the stand on one leg test also tests the person’s ability to listen, remember and comply with instructions. The theory here is a person who is impaired by alcohol will not be able to remember or comply with instructions.
Field sobriety tests done in DUI cases have some major accuracy issue. Among the issues are the conditions that these tests are done (which are not ideal for scientific tests), the problems with officer bias (officers basically looking for reasons to fail the test) and “failing” signs that can stem from the driver’s nervousness and not impairment.