Section 50. The armed forces of the United States and any part of the armed forces of the commonwealth parading or performing any duty according to law shall have the right of way in any street or highway through which they may pass, and drivers of vehicles in a military convoy, consisting of five or more vehicles, may drive such vehicles through an intersection of ways contrary to any traffic signs or signals regulating traffic at such intersection, if a police officer or duly authorized member of the military service is then stationed at such intersection to regulate traffic; provided, that the carriage of the United States mails, the legitimate functions of the police, and the progress and operation of fire departments shall not be interfered with thereby.