All ordinances of the city shall apply within land lying outside of the city limits that is owned or leased by the city, and any violation of an ordinance of the city occurring within the territorial limits of such land may be punished by the city to the same extent and with like effect as if the violation occurred within the corporate limits; provided, however, that this section shall not apply unless the common council of the city authorizes or ratifies by motion the erection, at any point at which a public road enters such land, of a sign, not less than eighteen inches by three feet (18" x 3') in size, containing a warning notice in bold letters that the area being entered is subject to the jurisdiction of the city. (1969 Code; 1986 Code)