As soon as the red lights come on behind you, the officer is looking for signs you are Driving Under the Influence. He is trained to document the reason for the stop. He is also looking to document anything unusual that happens from the time the officer activates his red lights to the time he arrests you for DUI.
There is even a Standardized Field Sobriety Testing Manual (SFST Manual). It covers everything the arresting officer is trained to observe. In my opinion, the Field Sobriety Test (FST) is nothing more than a balance and coordination exam. Many of my clients say that they can’t do the FSTs that are given on the side of the freeway, even if they were alcohol free and trying the tests in a well lighted, quiet room.
However, before you even get to the step in the process where you are standing by the side of the road, trying to tap dance on the head of a pin while the officer breathes down your neck and trucks are thundering by, the officer has watched closely for several things. The SFST Manual tells the officer that he should look for whether or not there is an attempt to flee; slow or no response to the emergency lights from the police car; any abrupt swerving; how the vehicle stopped; and whether or not your car hit the curb or some other object.