A person has complete freedom to choose whether or not to interact with the police officer. When a citizen initiates the contact, a choice is made. When an officer initiates the voluntary contact, the citizen still has a choice of whether to participate in that interaction. There are occasions in which a police officer initiates contacts that are not voluntary. Certainly a citizen has a choice whether to cooperate with the officer or not, and absent the choice to cooperate, the officer will decide what action is appropriate to try to gain compliance.
Most citizens choose to comply with the officer's directions.
Traffic violations are the most frequent contact under these conditions. While most citizens would prefer not to engage in a contact for a traffic infraction, most do so and the contact is completed with civility. There are circumstances in which an officer may use varying degrees of force to gain compliance. An officer may stop someone based on a reasonable suspicion that a person has committed a crime or is involved in criminal activity, or is about to be involved in criminal activity. An officer may also stop someone based on probable cause.