Members of the Police Department, Special Police, and Sheriff’s Department assigned to traffic duty are hereby authorized to direct all traffic in accordance with the provisions of this Article or in emergencies as public safety or convenience may require, and it shall be unlawful for any person to fail or refuse to comply with any lawful order, signal or direction of a policeman. Except in cases of emergency, it shall be unlawful for any person not authorized by law to direct or attempt to direct traffic. (See 625 ILCS 5/11-203)