There are different levels of certainty and circumstances that allow cops to do different things:
They got nothin': Cops can come up to you and ask you questions on the street without needing a reason (though you don’t have to answer). If you run in the opposite direction as soon as a cop approaches you though, that might give them…
Reasonable suspicion: Objective and articulable facts that a cop can interpret to mean something is up. The facts dictate what actions a cop can then take. For instance, if they see you swerving on the road, they can do a stop. But unless they have reason to believe you have a gun, they can’t do a full search for a gun. And if there’s no suspicion for drugs, then no frisk. The duration, force, and level of intrusion of the search must match the facts.
Probable cause: This is the evidence that brings reasonable suspicion to the next level. Probable cause is necessary to get a warrant ie corroboration or testimony. This could be suspended if there’s an emergency situation.
Administrative: These are the checkpoints cops set up on New Year’s Eve where they stop all the cars to see if people are drunk. They need to follow a general scheme and the intrusion has to be balanced against the public good.
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