To stop a vehicle, a police officer only needs a reasonable suspicion that the driver is committing, or has committed, a traffic infraction. Usually that suspicion arises from the officer's own observation of the driver and vehicle; however, police also have the authority to stop a vehicle based upon information from an another anonymous driver or pedestrian.
On its own, a simple traffic violation will not justify the search of a vehicle; a driver's excessive speed or other minor traffic infraction does not usually indicate that the driver is violent, and it does not give the police officer any reason to think that he is in danger of being assaulted.
However, if the officer has a reasonable belief that he or she might be assaulted by the driver, or if the motorist fails to produce a driver's license or produces a forged one, a search by the officer may be valid.
The officer's suspicion might be reasonable even if the officer makes a mistake of fact, for instance if their computer mistakenly reports that the driver's license is under suspension. However, if the officer does stop a driver based on a mistake of law, the officer's suspicion is not reasonable and the stop will usually be invalidated.
A pretextual traffic stop occurs when an officer uses the suspicion of a traffic violation as an excuse to stop a vehicle for another reason. Pretext stops may still be valid even if a reasonable officer would not have made the stop. Thus, the officer's ulterior motive for making the stop may not be relevant in determining the validity of the stop. Still, many state courts will not blindly accept the officer's pretextual traffic violation justification. For example, weaving and improper lane changes may not be sufficient to show the pretext of a traffic violation unless it is also shown that the motorist's driving posed a safety issue to another vehicle.
Roadblocks or sobriety checkpoints are permitted under the Fourth Amendment so long as they are conducted in a neutral or non-arbitrary manner, their intrusion on motorists is limited, and they further an important governmental or public purpose. There is no requirement that an officer have a reasonable suspicion of criminal activity to justify a stop at a roadblock.
Independent of traffic violations, if a police officer has a reasonable suspicion that a motorist is either committing or preparing to commit a crime, the officer is justified in stopping the vehicle.
Once police officers have lawfully stopped a vehicle, either because of probable cause for a traffic infraction or reasonable suspicion of criminal activity, they can:
- Order the occupants out of the vehicle
- Ask to see the driver's license, registration, and other relevant information, such as proof of insurance.
- Conduct a limited search to gain access to the vehicle identification number.
- Conduct a dog sniff (''canine sniff''), so long as the sniff does not extend the length of the stop.
- Take actions reasonably related to the original reason for stopping the vehicle, or related to suspicions that develop during the stop.
- Frisk for weapons if they have or develop a reasonable suspicion that the occupants may be armed or dangerous, and
- Search the vehicle if the stop provides probable cause for the officers to believe it contains illegal or stolen goods or evidence of a crime.