Members of the police department, and special police assigned to traffic duty, are hereby authorized to direct all traffic in accordance with the provisions of this Chapter, 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 case of emergency it shall be unlawful for any person not authorized by law to direct or attempt to direct traffic.