Whren v. U.S. [517 U.S. 806 (1996)]
Plainclothes officers in an unmarked car spotted a suspicious SUV with young Black occupants and temporary tags. The vehicle remained at a stop sign for an unusually long time, during which the driver appeared to be looking into the passenger's lap. When the officers made a U-turn in order to get a second look at the vehicle, it turned suddenly without signaling and reached an "unreasonable" speed. The officers caught up with the vehicle at a traffic light, and upon approaching the driver's side door spotted two large bags of crack in plain view. The defendants were convicted and appealed claiming that the officers' decision to stop them was motivated by an unsupported belief that they were involved in drug dealing, rather than by a desire to warn them about traffic laws.
The Supreme Court upheld the conviction, finding that for any seizure officers need only cite valid probable cause for any offense. The ruling rejected the defendants' argument that, because the officers' decision to stop them was motivated by an unsubstantiated suspicion of drug dealing and not a genuine concern regarding the manner in which the vehicle was being operated, the seizure was illegal under the Fourth Amendment. In addressing the defendants' argument that racial profiling is encouraged when officers are allowed to pull over motorists for reasons other then the actual crime being investigated, the Court argued that "there is no realistic alternative" to the general principle that probable cause validates a seizure.
What you should know about pretextual stops:
A "pretext" or "pretextual" stop is a stop in which the officer detains the citizen for a minor crime (i.e. traffic offense) because the officer actually suspects the person of involvement in another, mor significant crime (i.e. drug possession). The ruling in Whren v. U.S. demonstrates how easy it is for officers to do this. There are numerous minor infractions for which officers can legally pull over a car -- dirty license plate, broken tail light, changing lanes without signaling, loud muffler, etc. As such, officers frequently choose which cars to pull over based on suspicions that something more serious might be going on. Police officers make this decision based on age, race, and appearance. This is unconstitutional, but impossible to prove.
Therefore, the first defense against pretextual stops is to avoid violating any traffic laws. This includes, but is not limited to, obeying speed limits and traffic signs, signaling properly before making turns or changing lanes, and keeping your car in proper working order. If the officer can't form a convincing explanation for why you were pulled over, the seizure becomes illegal and any evidence found during the traffic stop can't be used in court. Of course, a legitimate traffic stop always remains a possibility, thus it is important to keep private items out of sight and, as always, never consent to searches.