It would be easy to say that each state maintains a state-wide police force that exists to enforce laws uniformly throughout that state. But it's not that simple, nor would it be true. There is a great deal more to state organizational structures than just troopers patrolling the interstates. Forty-nine of the fifty states maintain a uniform state-wide law enforcement agency. The fiftieth state, Hawaii, provides a state-wide agency for the coordination of one specific area of law enforcement.

Whether a state-wide police agency is called the state police, highway patrol, state patrol, troopers, militia, rangers, department of public safety, or another title, it is distinctive from the county sheriff's office by virtue of the fact that the operation is financed by the entire population of the state, as opposed to that of a single county. State agencies differ from local departments in the amount of territory that is considered to be within the jurisdiction of the state agency.

Some states have multiple enforcement agencies that address their efforts to a specific set of that state's laws and regulations. Some focus exclusively upon enforcing motor vehicle laws, while others enforce regulatory laws that deal with equipment or compliance issues in commercial operations. Still others focus exclusively on major crimes and misdemeanor offenses, or coordinate investigative efforts between agencies.

Regardless of the name employed, state police agencies are most commonly associated with the term troopers, and troopers are most often associated with those officers who patrol the state and interstate highway system in search of traffic violators. Pulling over drivers who broke the speed limit or who have expired inspection stickers often causes troopers to uncover more serious crimes. From the little violations often come the big cases. The important issue at hand is always probable cause, and the simple traffic violation often becomes the probable cause for the bigger case that follows.

Traffic violations give the patrol officers probable cause to stop vehicles. During the stop, a routine check of the driver's license and the vehicle registration and inspection are normal. Presuming that everything is in order, and that the individual driving the vehicle is not wanted for other offenses, a traffic summons may or may not be issued. But the violation that was first observed is the justification for the stop and all actions that follow.