Wis. Stat. § 346.40: Whistle signals.

(1) Whenever traffic is alternately being directed to stop and to proceed by a traffic officer using a whistle, such officer shall use the following whistle signals which shall signify as follows:

(a) One blast of the whistle means that all traffic not within the intersection shall stop.

(b) Two blasts of the whistle means that traffic which had been stopped prior to the one blast shall proceed through the intersection and that the traffic which was stopped by the one blast shall remain stopped.

(2) The traffic officer shall regulate the interval between the one and the 2 blasts so as to permit traffic that is legally within the intersection to clear the intersection.