(a) Whenever a traffic sign or traffic control device is placed in a position approximately conforming to the requirements of this code, it shall be presumed to have been placed by the official act or direction of lawful authority, unless the contrary is established by competent evidence.
(b) Any sign or traffic control device placed pursuant to this code and purporting to conform to the lawful requirements pertaining to it shall be presumed to comply with the requirements of this code unless the contrary is established by competent evidence.