WI318 is a current/future state highway being planned as the western Wasukesha bypass is being upgraded. It currently travels from US18 at Meadowbrook Rd and heads north to I-94. This segment has been signed as of 2018, I saw it yesterday on my way to Madison. I unfortunately was unable to take a picture since my phone was not ready(and I took WI16 back to Milwaukee which misses WI318), however one
AARoads user has posted a picture of the signage.In the future, WI318 will extend southerly down to WI59 by 2019, I will reply to this post when I see that WI318 has been extended.
Source 1(From here, use the link found in the References section at the bottom of the page. For some reason I cannot seem to find a way to link ftp websites.)