Wis. Stat. § 347.19: Visibility of clearance lamps and reflectors.

(1) Every reflector required by s. 347.16 to be displayed upon a vehicle shall be of such size and characteristics and so maintained as to be readily visible during the hours of darkness from all distances within 500 feet to 50 feet from the vehicle when directly in front of lawful upper beams of headlamps.

(2) Front and rear clearance lamps when lighted shall be capable of being seen and distinguished under normal atmospheric conditions during hours of darkness at a distance of 500 feet from the front and rear, respectively, of the vehicle.