Probable Cause
The Fourth Amendment also states that there must be "probable cause." This means that there is enough evidence to show that a crime has likely been committed. The police must have this evidence before any arrest or search. Any evidence found during the search does not count as probable cause. There are certain places and situations where people are searched or stopped without a warrant. Consider the airport where everyone who flies is searched. When you agree to fly, you give up some of your Fourth Amendment rights. Another example is a roadblock that tests for drunk drivers. When you drive on public roads you give up some of your Fourth Amendment rights. These searches are generally accepted by the citizens for their own safety and protection.