Being detained is another word for being under arrest. So says the US Supreme Court. A traffic stop is another word for being under arrest. So says the US Supreme Court. If you are free to go, you are not under arrest. If you are not free to go you are under arrest until such a time as you are released from custody. If the jailer kicks you out of jail because there's no more room at the inn on Saturday night, you have been released from Custody. If a traffic cop issues you a warning for going 50 in a 45, and then says "you are free to go," you have been released from custody.