The red flag that first triggers a traffic stop may not always be correct. Still, Wisconsin law enforcement officers are duty-bound to follow up on their suspicions and either conclude that nothing illegal is happening or stop a crime in progress.

When officers follow a logical course of action after the initial stop, the fact that their original hunch missed the mark might not be enough to stop an arrest for a separate offense, such as driving under the influence.

Reasonable suspicion and probable cause

According to FindLaw, the Fourth Amendment rights that protect citizens against illegal search and seizure rest on the concepts of reasonable suspicion and probable cause. Simply stated, law enforcement officers must make a concrete observation that suggests intoxication before they can conduct a vehicle stop and investigate.

This most often includes actual traffic violations like speeding or unusual behavior like erratic driving. However, officers can make DUI arrests after halting drivers for unrelated offenses if they uncover enough evidence, known as probable cause, during the stop’s normal progress. This means that while police cannot randomly search a car for alcohol containers, the odor of alcohol or slurred speech could lead to an arrest.

Expectation of privacy and routine traffic stop procedures

A Wisconsin Supreme Court appeal for a DUI conviction reported by the State Bar illustrates a plausible chain of events where an officer followed routine procedure and thus avoided violating the driver’s expectation of privacy, even after his investigation dissolved reasonable suspicion. The officer determined that a driver was operating a vehicle that belonged to an individual with a suspended license. Since the most logical conclusion was that the driver was the owner, he had enough reasonable suspicion to pull the car over.

However, as he approached the car, he discovered that the driver was of the opposite gender and obviously not the individual with the suspended license. At this point, the defense argued that since the officer’s suspicion was incorrect, he should have concluded the stop immediately because there was no longer reason to believe that the driver was breaking the law.

Yet, it is an accepted procedure for police to follow through with any traffic stop far enough to at least verify the driver’s identification. Thus, the Supreme Court determined that the occupant had no expectation of privacy that would allow him to refuse to speak to the officer. Their interaction led to the officer’s observation of intoxicated behavior, which gave him probable cause for a DUI arrest.