If police officers stop you, suspecting a DUI, they may ask you to step out of your car and perform a field sobriety test. The standardized form of this test is supposed to measure your eye's ability to track, as well as basic physical coordination.
While the line of thinking is that the test makes it easy to tell whether you have consumed enough alcohol to impair important functions, several factors make this test inaccurate. Unlike breath testing, the sobriety test is not covered by Tennessee's implied consent laws, so you should not incur the corresponding penalties for refusing.
How the test works
The major components of a standard FST are the horizontal gaze nystagmus, which tests the eye's ability to focus and track; the walk and turn and the one-leg stand. Many people do not realize that the test actually begins when the officer starts giving the instructions. Visible inattention or failure to follow directions exactly will count against you in the officer's assessment.
Why you do not need to be impaired to fail
If you think there could be a million reasons why a person might fail any part of the test without having taken even a sip of alcohol, you are correct. Many types of medical conditions, as well as some medications, can affect a person's balance or eye focus. Uncomfortable shoes or even simple tiredness can lead to a lack of balance.
People mishear or misinterpret instructions because they have a hearing impairment, do not speak English or become extremely nervous, as many people do when interacting with law enforcement. A high level of noise nearby can also interfere with your ability to hear the instructions properly.
No objective way to know if you passed or failed
In addition, the decision whether someone passed or failed an FST can be highly subjective. Although officers are supposed to receive training in administering and evaluating the FST, there is not a way for these results to be completely independent from a particular officer's individual perceptions.
What to do
Nevertheless, a failed test can add to the weight of evidence against you. For this reason, you should courteously decline to take it. If you did take and fail the test, an experienced attorney may still be able to challenge this and other evidence against you.