Vehicles while discharging freight may back to the curb and shall occupy as little of the street as possible, leaving available not less than ten feet of the street for free movement of vehicular traffic at all times. If such vehicle is of the semitrailer type, the tractor part shall be turned so as to stand parallel with the curb and headed in the direction of the traffic, and shall be so parked only during the time necessary for the actual loading or unloading thereof.