United States forces or troops, or any portion of the Virginia National Guard, parading or performing any duty according to law, or any civil defense personnel performing any duty according to law, shall have the right-of-way in any highway through which they may pass. Such passage, however, shall not interfere with the carrying of the United States mails and the legitimate functions of police and fire fighters or with the passage of emergency vehicles as defined in § 46.2-920.