Proving that a person is intoxicated without a blood or breath test is not an exact science. There are three tests that officers use to determine if you are impaired. The problem with these tests is that people who are completely sober can fail them. The tests include standing on one leg, following a pen's movement with your eyes and walking and turning.
The problem with these tests is that there is a lot of room for errors. Here are a few examples of how each of these tests could lead to an innocent person being accused of driving while intoxicated.
The one-leg stand
With the one-leg stand, a person has to stand on one leg and balance for a period of time. The trouble with this is that some people have medical conditions or physical problems that make it impossible for them to balance easily. They could fail this test even if completely sober.
The horizontal gaze nystagmus test
The horizontal gaze nystagmus test is a little harder to fail without intoxication. However, people who have seizures or problems with their eyes may fail the test because of an unsteady gaze.
The walk-and-turn test
Finally, the walk-and-turn test has similar problems to the one-leg stand. Law enforcement officers ask people to walk heel-to-toe, which is unnatural. Physical conditions and medical problems can make this test difficult to pass.
As you can see, field sobriety tests are not foolproof. If you fail a field sobriety test, there is still a chance that you can be acquitted of a DUI. Your attorney can help you present evidence of physical disabilities or illnesses that made it difficult for you to pass the above tests.