All signs, banners, flags, or advertisements mentioned in Section 13.24.020 installed, erected and maintained over and across any public street or alley shall be installed, erected and maintained at a distance of not less than fifteen feet above any such public street or alley and shall be so installed, erected, constructed and maintained as not to endanger traffic. Such signs, flags, banners or advertisements shall be erected and maintained in a substantial manner and attached and securely fastened to a support or structure sufficiently strong to prevent the collapse or falling thereof or to in any way endanger the public. (Ord. 840, 1939)