The Fourth Amendment protects against “unreasonable” searches and seizures, but what the courts consider “unreasonable” has evolved and shifted over time. One overarching trend over the last few decades is that police officers have been given significant leeway, and usually the benefit of the doubt, to stop and search individuals in various ways. Under federal law, police can search people and their property under warrants that turn out to be invalid, as long as they are acting in the good faith belief that the warrants are valid. The Supreme Court has held that police can validly arrest someone under a law that is later ruled unconstitutional. And police can stop or search someone whom they suspect of illegal activity, even, in some circumstances, if the suspicion was based on inaccurate or faulty information.
One common area where Fourth Amendment rights come into play is traffic stops. The police cannot stop a driver for no reason; such a stop must generally (with narrow exceptions such as sobriety checkpoints) be based on a reasonable suspicion that the driver has violated the law. However, courts routinely uphold stops under the Fourth Amendment when an officer’s attention is drawn to a car based on a hunch (or even racial profiling). A common police tactic is to follow a car until it commits some type of traffic violation or other infraction, and then use that as the basis to stop the car and make further inquiries. Courts have routinely refused to look past this gambit, holding that it is permissible to stop someone if there is in fact a traffic violation, no matter what the officer’s “real reason” for a stop was.