Police who suspect that a driver is impaired often administer field sobriety tests in order to gather additional incriminating evidence against the driver. These tests include the alphabet test, horizontal gaze nystagmus, 9-step walk and turn, and the 30-second one leg stand. Other possible tests are touching the finger to nose or counting backwards. Some of these tests have been shown to be somewhat reliable as a means to detect impairment, provided that the tests are administered correctly by the officer.
A common issue in court is whether the officer correctly instructed the subject, and objectively scored the test. Since the accuracy rate of the tests (per the officers’ training materials) is less than 70% even if administered perfectly, much is gained by defendants who are able to show that the tests themselves were flawed, or that the officer administered the tests in a manner different from his or her training.
Most police officers will testify that they did not decide to arrest until the field sobriety tests were completed, and admit on cross-examination that there was not probable cause until the defendant performed poorly on the tests. As a result, if the jury has doubt concerning the accuracy of the tests themselves, a defendant may be well on his way to an acquittal.