Wis. Stat. § 194.32
No common motor carrier of passengers shall operate any passenger-carrying bus over any public highway of this state with any trailer or semitrailer attached except for an articulated bus as defined in s. 340.01(2m). No interurban motor bus which is of a double-decked open-roof design shall be operated upon the public highways under the authority of this chapter. As used in this section an interurban motor bus is deemed to be of a "double-decked open-roof design" when passengers are carried therein on an upper level throughout the length of the bus over passengers on a lower level throughout the length of the bus and the bus roof does not extend throughout the length of the bus or is not permanently enclosed with rigid construction.
Wis. Stat. § 194.32