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A traffic stop, commonly called being pulled over, is a temporary detention of a driver of a vehicle by police to investigate a possible crime. In constitutional law in the United States, a traffic stop is considered to be a subset of the Terry stop; the standard set by the United States Supreme Court in Terry v. Ohio regarding temporary detentions requires only reasonable suspicion that a crime has occurred or is about to occur.
A stop is usually accomplished through a process known as "pulling over" the suspect's vehicle. Police vehicles (except those used by undercover personnel) traditionally have sirens, loudspeakers, and lightbars that rotate and/or flash. These devices are used by the officer to get the attention of the suspect and to signal that they are expected to move over to the shoulder and stop.
These devices are also typically equipped on other emergency vehicles such as fire trucks and ambulances, and in all cases, such signals and the laws requiring that other vehicles pull to the shoulder allow the emergency vehicles to pass other traffic safely and efficiently when responding to emergency situations. In the case of a traffic stop, the officer pulls the patrol vehicle behind the subject vehicle as it stops instead of proceeding past as he or she would during emergency response.
Depending upon the severity of the crime which the officer believes to have occurred, the officer may arrest the suspect, either by taking him or her to jail, or check for any outstanding warrants before issuing a citation also called a Notice to Appear or summons in some jurisdictions, which is essentially a traffic ticket. In some cases, officers may choose to simply issue a verbal or written warning.
Traffic stops are inherently dangerous for police officers, many of whom patrol and conduct stops alone. Officers typically take steps to protect themselves from passing traffic such as using their own car as a shield and/or approaching the suspect vehicle on the passenger side.
Many states have enacted laws requiring freeway traffic approaching the police vehicle to merge over to the left, leaving an entire lane as a buffer zone for the officer. According to FBI statistics, more officers are killed or injured annually during the course of a traffic stop than at any other time excluding vehicle accidents and effecting arrests.
A "felony" or "high-risk" traffic stop occurs when police stop a vehicle which they have strong reason to believe contains a driver or passenger suspected of having committed a serious crime, especially of a nature that would lead the police to believe the suspect(s) may be armed (such as an armed robbery, assault with a weapon, or an outstanding felony warrant for the registered owner). In a high risk stop, officers attempt to provide for everyone's safety by issuing instructions to maintain absolute control over every step of the proceedings. They will have additional officers on scene for back-up, often waiting for additional officers to join up before initiating the stop. They will typically have their weapons drawn, and stay back from the suspect vehicle, using their patrol cars for cover. If there is no choice but to make the stop on a busy street, then they will often stop traffic. They will address the driver and any passengers over the PA speaker of the patrol car, typically instructing the driver to turn the engine off, remove the keys from the ignition, and sometimes toss them out the window. They will instruct the occupants, one at a time, to exit the vehicle with empty hands showing, place their hands on top of or behind their heads, walk backwards some distance, and then lay flat on the ground, where they will remain until all occupants have done likewise, at which point officers will move up, apply handcuffs, do a body search and then secure the suspects in the patrol cars. The vehicle is then typically searched for weapons and other evidence.
The Supreme Court has held that an officer who stops a vehicle as part of a routine traffic stop has the authority to order the driver to exit the vehicle, as well as to order any passengers to exit the vehicle.
Controversy in the United States 
In the United States, traffic stops have been criticized for their use in police dragnets to check compliance with laws such as those requiring the use of seat belts or those forbidding the possession of narcotics.
Some people have objected that the tactic violates the United States Constitution; the Fourth Amendment to the Constitution, part of the Bill of Rights, contains a provision against unreasonable search and seizure. Typically police must either have probable cause for a search or get a warrant from a judge specifying a particular individual by name or get a "John Doe warrant" with a specific description.
In Delaware v. Prouse, 440 U.S. 648 (1979), the United States Supreme Court ruled that the police stopping vehicles for no reason other than to check the drivers' licenses and registrations was unconstitutional.
In New York v. Belton, 453 U.S. 454 (1981), the United States Supreme court ruled that when a police officer has made a lawful arrest of a driver, he may search the passenger area of the vehicle without obtaining a warrant.
In Illinois v. Caballes, 543 U.S. 405 (2005), the United States Supreme Court held that a dog sniff, conducted during a concededly lawful traffic stop that reveals no information other than the location of a substance that no individual has any right to possess, does not violate the Fourth Amendment.
In Arizona v. Gant, (2008), the United States Supreme Court ruled that an officer must demonstrate a threat to their safety or a need to preserve evidence related to the crime of arrest in order to search a vehicle pursuant to an arrest, distinguishing New York v. Belton.
See also 
- LaFave, Wayne (August 2004). "The 'routine traffic stop' from start to finish: too much 'routine,' not enough Fourth Amendment". Michigan Law Review 102 (8): 1843–1906. Professor LaFave points out that most courts have treated traffic stops like Terry stops, but the U.S. Supreme Court itself has never squarely decided the issue of whether traffic stops require probable cause or the lesser reasonable suspicion standard of Terry.
- per curiam opinion. "Pennsylvania v. Mimms, 434 U.S. 106 (1977)". Cornell Law. Retrieved February 9, 2012.
- Chief Justice William Rehnquist. "Maryland v. Wilson, 519 U.S. 407 (1997)". Cornell Law. Retrieved February 9, 2012.
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