No person shall, without lawful authority, possess, or attempt to or in fact alter, deface, injure, knock down, or remove any official traffic-control device or any railroad sign or signal or any inscription, shield, or insignia thereon, or any other part thereof. A person who voluntarily notifies a law enforcement agency that the person is in possession of such an article, and who returns the article within ten days after gaining possession thereof, shall not be subject to prosecution for such possession.