Mo. Rev. Stat. § 300.260: Signs required at through streets.

Whenever any ordinance of the city designates and describes a through street it shall be the duty of the city traffic engineer to place and maintain a stop sign, or on the basis of an engineering and traffic investigation at any intersection a yield sign, on each and every street intersecting such through street unless traffic at any such intersection is controlled at all times by traffic control signals; provided, however, that at the intersection of two such through streets or at the intersection of a through 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 city traffic engineer upon the basis of an engineering and traffic study.