Wis. Stat. § 346.39: Flashing signals.

Whenever flashing red or yellow signals are used they require obedience by vehicular traffic as follows:

(1) Flashing red (stop signal). When a red lens is illuminated with rapid intermittent flashes, operators of vehicles shall stop before entering the nearest crosswalk at an intersection or at a limit line when marked, or, if none, then before entering the intersection, and the right to proceed is subject to the rules applicable after making a stop at a stop sign.

(2) Flashing yellow (caution signal). Subject to s. 346.37 (1) (e), when a yellow lens is illuminated with rapid intermittent flashes, operators of vehicles may proceed through the intersection or past such signal only with caution.