A police officer who is patrolling an area will watch for signs that something is amiss. This can be just about anything that is out of the ordinary. One of the activities the police are going to watch for as they ride around is drunk driving. Drivers who are too impaired to operate a vehicle pose a risk to other people on the road.
When ready to conduct a stop for suspected drunk driving, the officer needs reasonable suspicion that the driver is impaired. This can come from seeing common signs of drunk driving like swerving, stopping suddenly or speeding. Once the officer stops the vehicle, the driver's impairment level has to be determined. This can be done by conducting a field sobriety test.
Only some tests are standardized
Not all test are part of the standardized field sobriety test. There are only three tests that are part of the standardized battery of tests. These are the walk and turn, the one leg stand, and the horizontal gaze nystagmus test. Officers must be trained to conduct the test and interpret the results. They must be performed in a controlled environment and are only appropriate in certain situations.
Some officers will conduct other tests, but those aren't part of the battery endorsed by the National Highway and Traffic Safety Administration. These non-standardized aren't admissible in court. Still, some jurisdictions use them as a precursor to blood alcohol concentration tests.
Important facts about the SFST
A police officer has to look for specific signs of impairment when a person is doing the tests. There are sometimes factors outside of impairment that can skew the results to make it appear that a person is impaired. For example, a person who has a brain injury might not be able to balance for the one leg stand and might have more jerking in the eye than what is considered normal for the horizontal gaze nystagmus test.
The possibility of other factors impacting the test and the chance of an officer misinterpreting the test are reasons why some people don't believe that these tests are a good idea. There are several factors that might lead to a field sobriety test being called into question in court. Even things like the terrain and oncoming traffic can have a negative impact on the quality of the test, so all of these should be explored when you are assembling a defense strategy.