Wis. Stat. § 194.09
Each motor vehicle operated by a common motor carrier of property or of passengers, a contract motor carrier or a private motor carrier shall be plainly marked in such manner as the department may prescribe, so as to identify such motor vehicle as being operated pursuant to this chapter. This section does not apply to any farm truck or dual purpose farm truck combined with any semitrailer or farm trailer, or any vehicle combined with a horse trailer, if the vehicle combination's gross combination weight rating, registered weight, and actual gross weight do not exceed 26,000 pounds, the vehicle combination does not include a commercial motor vehicle described in s. 340.01(8) (c) or (d), and the vehicle combination is operated solely in intrastate commerce.
Wis. Stat. § 194.09