Once stopped, the officer must believe you are intoxicated or impaired by drugs or alcohol.   Quite often, police reports reflect that the individual had glassy eyes, admitted drinking, smelled of alcohol, had slurred speech, stumbled, staggered, or fumbled while attempting to produce documentation. Any combination of these indicators may provide the officer probable cause to believe a driver is impaired.
If an officer has reason to believe an individual is driving while intoxicated the officer will request that the individual submit to a chemical test, (the Alcotest breath reading).  Failure to submit to the chemical test will result in a charge of Refusal which has consequences similar to and in some instances more severe than a first offense DWI.