Whether an officer may pull over a motor vehicle in this state is broadly governed by the Fourth Amendment to the U.S. Constitution, which guarantees “the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures.”
The U.S. Supreme Court and the Missouri Supreme Court have held that an officer may not conduct an investigatory stop absent some “reasonable suspicion that criminal activity is afoot.” In other words, the officer must reasonably believe that the person has been, or is about to be, engaged in some form of criminal activity.
So therefore an officer may pull someone over for speeding, or running a stop sign, or perhaps even driving erratically. But the officer must be able to articulate a reason for the stop. If not, a good criminal attorney should file a motion to suppress all evidence that came from the illegal and unconstitutional traffic stop.