Police often set up roadblocks—also called checkpoints—where they stop and inspect 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. The Court relaxed the normal requirement for probable cause because border control is such an important issue.
Requirements for a Valid Roadblock
For a checkpoint to be valid, the police must follow the same procedures with respect to all motorists on a route; they cannot discriminatorily target any particular driver. Even if the police follow the same procedures for all drivers, a roadblock may still be illegal if its purpose is not closely tied to highway safety and instead is directed only at general crime control.
For example, a roadblock at which police stop only the fourth car would probably pass muster; one in which police stopped only those drivers whom they thought might have drugs in the car would not be legal.
Stopping Cars to Look for Undocumented Aliens
Police may legally operate a roadblock whose purpose is to catch undocumented aliens, as long as every car (or every second, third, fourth, or fifth car, and so on) is stopped. (United States v. Martinez-Fuerte, U.S. Supreme Court, 1976.) The usual requirement of probable cause is relaxed due to the government's interest in stopping illegal entry, an interest that overrides the privacy interests of the vehicles' occupants. Police may not stop only those cars whose occupants they suspect may be in the country illegally. (U.S. Brignoni-Ponce, U.S. Supreme Court, 1975.)
Stopping Cars to Control Crime
Although it's legal to set up and operate a roadblock for border control (as long as police don't target individual drivers), 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, stopping all cars in the hope that the occupants will be caught violating drug laws is invalid. The use and possession of illegal substances are examples of ordinary criminal wrongdoing, having nothing in particular to do with highways and cars. (Indianapolis v. Edmund, U.S. Supreme Court, 2000.)