 while the route past I-94 is classified as a "Non-Interstate STRAHNET Route"

WIS 21 begins at the intersection of WIS 16, WIS 71 and WIS 27 The route then follows WIS 16 and WIS 71 east for about one half of a mile before turning north off the concurrency.WIS 21 proceeds northeast, following the La Crosse River and passes through Fort McCoy.The route then turns eastward and follows alongside the Tarr and Sparta Creeks, passing through Tunnel City.