The cases date back to the 1920s , when Prohibition created an illicit trade in alcohol. Over time, these cases strengthened the definition of reasonableness. First, because a car is mobile, an officer may decide to search a car on the spot under the reasonable assumption that a suspect will flee before a warrant can be issued. Second, because automobiles travel on public streets, the court has argued that cars convey only limited privacy to the passengers inside. Therefore, officers have a much broader latitude of reasonableness for warrantless car searches and driver arrests than they might for similar actions at a private home or office.
Drivers still have some protections under the Fourth Amendment.
Supreme Court precedent has upheld the doctrine of probable cause. An officer cannot stop a car simply because he believes the driver or a passenger intends to engage in a crime. But probable cause and reasonableness are difficult to quantify. Ultimately, they rest on a series of assumptions about what kinds of drivers are likely to be engaged in criminal activity. It should come as no surprise that race, gender and class contribute to the appearance of suspicious behavior.