Wisconsin Statutes Vehicles (Ch. 340 to 351) § 347.40. Mirrors

(1) No person shall operate any motor vehicle upon a highway unless such vehicle is equipped with a mirror so located as to reflect to the operator a view of the roadway for a distance of 200 feet to the rear of such vehicle.

(2) No person shall operate on a highway any school bus having a passenger-carrying capacity of 10 or more persons including the operator unless such bus is equipped with at least one mirror which is 7 inches in diameter so located as to enable the operator to see a reflection of the road from the entire front bumper forward to a point where direct observation is possible.

(3) No person may operate or permit the operation of any motor bus on a highway unless the bus is equipped with 2 outside rearview mirrors, one to the right and one to the left of the operator. Each mirror shall have not less than 50 square inches of unobstructed reflective surface and shall be firmly supported and adjustable to give the operator a clear view past both the right and left rear of the bus.