The field sobriety tests are tests administered by law enforcement to determine sobriety from the side of the road. These tests were researched in the 1970s by the Southern California Research Institute and Standardized by the National Highway Traffic and Safety Administration. They have been approved to date as a ‘reliable’ method of establishing probable cause for arrest under the statute 169.20 (driving while impaired) in the state of Minnesota.
The Horizontal Gaze Nystagmus (HGN) – 88% reliable
The HGN Field Sobriety Test measures the response of the Nystagmus which is a rapid, oscillating, and involuntary movement of the eye. The Nystagmus is exaggerated with the use of alcohol and particular types of drugs. The officer checks the Nystagmus of your eye with a pen or similar tool in which you will follow only with your eyes at a comfortable distance. The officer is trained to examine each eye for Nystagmus as the officer moves the stimulus from side to side.
With this test the examiner is looking for three indicators in each eye. First your eyes must be moving smoothly across the field of vision. If your eyes wobble involuntarily, HGN might be present. Second, the officer is looking for whether the eye is consistently jerking involuntarily when it is held at the corner of your eye known as Maximum Deviation.
Lastly, the officer will notice a jerking of the eye perhaps around 45 degrees from the center and will continue to find the angle that onsets the jerk known as the angle of onset. Four or more clues within the HGN test will give an officer probable cause that you have most likely consumed something to impair your driving ability.
Walk and Turn Test (WAT) – 79% reliable
The WAT test is referred to as a Divided Attention test because it is structured to divide your physical attention, concentration of your physical performance, from your mental ability to follow directions. In theory, a sober person is more inclined to pass, as alcohol or drugs hinder your ability to concentrate on both at once.
As with these next field sobriety tests, the officer must stand away motionless as to not disrupt your balance and concentration. The suspect is assumed to be safe from traffic and must walk nine steps heel to toe with arms on their side along a clear line on the road -turn around and walk nine steps back in the same fashion. The officer administering the test should show you how to do it first. Failure to stay on the line, maintain your balance, or follow any instructions of the test will count against you in the scoring procedure.
One Leg Stand (OLS) – 83% reliable
The first stage of the OLS test requires the officer to instruct the suspect on how it is done. The officer should demonstrate field sobriety tests and clarify that the suspect understands the instructions. In this test the suspect will be told not to begin until instructed to do so.
Then required to balance on one leg raising the foot about six inches off the ground, the suspect is instructed to count to thirty starting with ‘one-one-thousand, two-one-thousand, etc., etc.’ The officer will time the suspect watching for swaying of any kind, the use of hands for balance, hoping around to maintain balance and putting the foot down. These mistakes will count against you, and after thirty seconds the officer can decide to arrest you for having been driving while impaired.
Although field sobriety tests are used to establish probable cause, they are rarely used as proof of actual impairment.