The manager may designate and describe arterial streets or highways. When so designated, it shall be the duty of the superintendent of public works to place and maintain a “stop” sign on each and every street or highway intersecting such arterial street or highway or intersecting that portion thereof described and designated as such unless traffic at any such intersection is controlled at all times by traffic control signals; provided, however, that, at the intersection of two such arterial streets or highways or at the intersection of an arterial street and a heavy traffic street not so designated, “stop” signs shall be erected at the approaches of either of said streets as may be determined by the manager on the basis of a traffic study.