Officers use field sobriety tests to evaluate the balance, agility and attention level of drivers when they suspect intoxication or impairment.
When pulled over for a suspected DUI, officers will likely ask you to perform three tests.
Three standard tests
For the Horizontal Gaze Nystagmus test, the officer will hold a finger or other small object about 12 inches in front of your face and move it slowly from one side to the other. You must follow the object with your eyes without moving your head. The eye involuntarily jerks when it gazes to the side. The jerking is more exaggerated in an intoxicated person.
When an officer asks you to perform the One-Leg Stand test you will stand with one foot raised six inches off the ground for 30 seconds. Any swaying, dropping your foot or using your arms for balance is a sign of possible intoxication.
The Walk-and-Turn test requires you to walk nine steps, heel to toe, in a straight line before turning on one foot and walking back in the same manner. This tests your ability to complete tasks with divided attention and is easily completed by unimpaired people.
The usefulness of the tests
Field sobriety tests are not infallible and are not scientific. They are rarely used as evidence in court. They are primarily used by officers to determine if further testing should occur. In some cases, the officers may use the field sobriety test results as grounds for arrest.
Reasons you may fail unrelated to intoxication
Field sobriety tests evaluate the physical control a person has over their body, as lack of control is often an indicator of intoxication. However, people with physical disabilities or illnesses such as an inner ear infection that can affect balance may fail these tests completely sober.
When officers decide to conduct further testing, they may request a blood or breath sample. A blood sample will require your consent or a warrant. However, there is an implied consent law in South Dakota that states driving on the road is giving your consent to have your breath tested. Refusing to comply will cause a license suspension for one year.