Illinois Route 35 is the tiniest route in Illinois, and it lies in the very northwest corner of the state. Ironically, the extension of Wisconsin's longest highway into Illinois constitutes Illinois's shortest route. That Wisconsin highway is Trunk Highway 35, but the southern end of its course is only two miles into the state of Illinois, at US Route 20 in East Dubuque.
The northern end of Illinois 35 is, predictably, the Wisconsin border, within two miles of the Mississippi River. (My visit to the site revealed no end marker, so I took this southward-facing photo instead):
Illinois 35 travels a lousy two miles before ending at a sort of interchange in East Dubuque, with two east-west city streets serving as ramps that take travelers up to US Route 20.