As a general rule, an officer must have reasonable and articulable suspicion that someone has, or is about to engage in criminal activity in order to legally stop them. Much like many other legal terms, the phrase "reasonable and articulable suspicion" does not, in a practical sense, answer the question of when an officer can pull someone over.
In some situations, it is apparent that an officer is allowed to stop a vehicle. For example, when a vehicle is exceeding the posted speed limit, swerving in and out of a lane, or driving without its headlights at night. It may appear that in order for an officer to stop a vehicle they must observe a traffic violation. However, in some situations, an officer can make a legal traffic stop without directly observing a traffic infraction or any other illegal act. This was recently demonstrated in an unpublished opinion from the Minnesota Court of Appeals in State of Minnesota v. Jason James Johnson.
In Johnson, a man pulled into a school parking lot, parked, and turned off his headlights at approximately midnight. A police observed this and pulled into the parking lot to investigate. When the squad car pulled in, Johnson turned his headlights back on and began to drive away. The officer activated her lights and stopped Johnson before he exited the parking lot. After questioning and performing field sobriety tests, Johnson was arrested on suspicion of DWI. Before trial, Johnson's motion to suppress and dismiss was denied and he later plead guilty, reserving his right to appeal.
Johnson challenged the legality of the stop and moved to have all of the evidence thrown out as a result. Johnson argued that he should not have been stopped because he did not commit any illegal driving conduct and was not a known suspect in any criminal activity. The State argued that Johnson's location, the time of night, previous suspicious activity in the area, and Johnson's evasive behavior all combined to provide officers with reasonable and articulable suspicion of criminal activity. The Court of Appeals agreed with the State and ruled that under the totality of the circumstances, the officers had reasonable and articulable suspicion to justify a stop.
The Johnson ruling demonstrates that reasonable and articulable suspicion is not a high bar to clear. Officers do not need to be able to show very much information regarding suspicious activity to lawfully stop someone. However, this does not mean that you should assume that a stop was lawful. In fact, in Johnson, a Carver County prosecutor almost did not file charges because they did not feel the stop was supported by reasonable and articulable suspicion. In short, the concept of reasonable and articulable suspicion is complex and requires very careful analysis by an experienced attorney. If you think you may have been unlawfully stopped, you should consult with a knowledgeable and experienced lawyer.