Nearly everything you read about DUI traffic stops will tell you that police officers cannot place you under arrest based on a feeling or suspicion. They must have probable cause in order to make an arrest.
How do police establish probable cause for a DUI arrest? Other than their observations of your driving and behavior, they do so through administering field sobriety tests. Since authorities use the results as evidence of impairment, you would think the tests are objective. Unfortunately, you would be wrong.