Wis. Stat. § 86.185: Populous counties; power over county trunk highways.

Counties having a population of at least 150,000 may exercise any corresponding power over county trunk highways that is conferred upon cities in the widening or improving and maintaining of state trunk or county trunk highways, the constructing or laying of water pipes, sewers and all other public service facilities therein, and in the levying of special assessments and issuing of assessment certificates and special improvement bonds.