Wis. Stat. § 66.0431: Prohibiting operators from leaving keys in parked motor vehicles.

The governing body of a city, village or town may by ordinance require every passenger motor vehicle to be equipped with a lock suitable to lock either the starting lever, throttle, steering apparatus, gear shift lever or ignition system; prohibit any person from permitting a motor vehicle in the person’s custody from standing or remaining unattended on any street, road, or alley or in any other public place, except an attended parking area, unless either the starting lever, throttle, steering apparatus, gear shift or ignition of the vehicle is locked and the key for that lock is removed from the vehicle; and provide forfeitures for violations of the ordinance. This section does not apply to motor vehicles operated by common carriers of passengers under ch. 194.