The department shall designate the street or streets which shall constitute the primary road extensions in any city of the state, which city is separated from the remainder of the state by a river more than five hundred feet in width from bank to bank. The laws of this state relating to the construction, reconstruction, or maintenance of the extensions of primary roads in cities, and to the purchase or condemnation of right-of-way for those primary roads, and to the expenditure of primary road funds thereon, shall apply to the roads or streets designated under this section, the same as though said community were not so separated from the rest of the state.