Yes, cops can sit outside a bar to monitor for possible illegal behavior such as drinking and then driving. As a society we want our police to be at places where possible crimes are going to be done. As part of police training, I believe the academy suggests such investigation skills be employed.
Yes, a cop can wait outside of a bar for as long as they like. However, they cannot just look and 'get' people for DUI. Unless the persons exiting display signs of intoxication and then drive a car, or commit traffic offenses after leaving, they cannot be stopped by the police.
Unfortunately, yes cops can wait outside of bars. It's ironic that people in bars are usually there in order to drink alcohol, which often leads to intoxication. If you don't want to be arrested for DWI, then simply have a designated driver with you whenever you go to a bar. Or don't drink while there. Otherwise, when you walk out of a bar and get into a car, you are likely to be a target.
The short answer is yes, police can sit outside bars whenever and for however long they like. But a person doesn't just "get" a DUI like a door prize. A person must show signs of intoxication. Police will watch for steady or unsteady movement and slurred speech, bloodshot and glassy eyes. If someone exhibits these signs then yes, the person may be arrested for a DUI if they got to a car.
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