Wisconsin: Questions about legality of yard signs calling for recall of Governor

Wauwatosa is debating whether "Recall Walker" signs dotting lawns in Wisconsin are a illegal -- there is a time limit on how far in advance and election sign can be put up. This article claims that the  DOT has a rule that signs are allowed up on state highways for only 45 days prior to an election, and must be down within a week. Not sure what that has to do with yard signs, but they are looking into it.