The longest Business US 20 in Illinois runs from just west of Rockford to just east of Belvidere. The route begins just west of Weldon Road and runs as State Street into downtown Rockford, where it is split into two one-way streets (Chestnut Street and Walnut Street for eastbound traffic, Jefferson Street for westbound traffic). The route continues east along State Street all the way through Rockford and becomes a two-lane highway between Rockford and Belvidere. Business US 20 is then routed in an outer loop around the city of Belvidere before merging back with US 20 near Genoa Road and I-90.