The original US Route Marker in Illinois was shield shaped with a separator line at the top. During the 1950's and early 1960's the separator line was dropped as was the state name. Eventually Illinois, along with most of the rest of the country, adopted the current US highway marker, a white shield surrounding a black number enclosed on a square blank. Several routes proposed have never been built, and some may eventually be built. Miles and some endpoints shown on 2 digit Interstates here come from the Zzyzx Interstate Page , an amazing reference to 2 digit Interstate highways.
Interstates are the main routes between cities, and are divided multilane highways.
Most are 2 lanes in either direction rural areas, but can swell to 6 or more in large cities. Interstate numbers are on most freeway and tollway miles in Illinois. Interstate numbers may exist in multiple states and are occasionally reused. Interstates were usually funded by a partnership of the State and Federal governments. Reconstruction of existing and building of new roads are financed on an individual basis, usually with some federal money.