Police often set up roadblocks—also called checkpoints—where they stop and inspect all (or almost all) drivers and vehicles passing along a road. Because the police typically lack probable cause to believe that any particular driver who is stopped has broken a law, checkpoints could violate the Fourth Amendment. But the U.S. Supreme Court has ruled that as long as the police follow certain protocols, the practice is valid. For example, suspicionless stops at and around the U.S border are often valid.
Stopping Cars to Look for Undocumented Immigrants
Police may stop motorists not only at actual border crossings, but also at checkpoints that are somewhat removed from the border. But the rules differ depending on where the stop occurs. For much more information on this area of law, see Search and Seizure at and Around the U.S. Border.
Stopping Cars to Control Crime
Although it's legal to set up and operate a roadblock for border control, it is not legal to do so for general crime control. Roadblocks must be established in order to serve a purpose closely related to a particular problem associated with automobiles and their mobility. For example, the U.S. Supreme Court once ruled that a vehicle-checkpoint system designed to "interdict" unlawful drugs violated the Fourth Amendment. The Court based its ruling on the conclusion that the primary purpose of the checkpoint program was "indistinguishable from the general interest in crime control." (Indianapolis v. Edmund, U.S. Supreme Court, 2000.)
Stopping Cars for Highway Safety
But other, highway-safety checkpoints may be legal. For instance, sobriety checkpoints can justify vehicle stops even where officers have no reason to believe each stopped driver is operating under the influence. (See Sobriety Checkpoints.)