For the most part, drivers in Wisconsin cannot be stopped on suspicion of drunk driving unless a police officer has a good reason to suspect that the driver is impaired. This is called reasonable suspicion, and it gives a police officer the right to detain someone and perform a brief investigation.
An officer could consider erratic driving such as drifting, weaving, sudden stops or unusually slow speeds to be cause for suspicion of DUI. It is not actually necessary for a driver to be driving for a police officer to have a reason to stop them on suspicion of operating while impaired. For example, if a driver gets out of his or her car and appears to be intoxicated, an officer could detain the driver and proceed with sobriety tests.
While reasonable suspicion may be enough to detain someone, this standard is not enough to warrant an arrest on a drunk driving charge. For that, an officer must have probable cause. Probable cause means that the officer has enough evidence to believe that a person has committed a crime. In the case of drunk driving, this evidence could be obtained by the officer by giving the driver a breath or sobriety test.
The difference between reasonable suspicion and probable cause is a matter of the likelihood that a person committed a crime. Reasonable suspicion means that there is enough evidence to suggest that someone might have committed a crime, while probable cause means that there is enough evidence to suggest that someone probably committed a crime.
Without probable cause for an arrest or even without reasonable suspicion to warrant a traffic stop, a police officer could be found to have acted improperly in stopping or arresting someone for drunk driving. When this happens, the charges could be reduced or even dropped, even if the driver was drunk.