^Northbound trailblazer at Illinois 176 in Crystal Lake, June 2020.
Illinois Route 31 runs north-south through the Fox Valley area of the far western Chicago suburbs, and extends north through McHenry County, almost to the Wisconsin border. From Oswego northward, it follows the west side of the Fox River, whereas Illinois 25 follows the east shore of that river.
The north end of Illinois 31 is at US Route 12 in Richmond; US Route 12 continues straight north from here to Wisconsin:
In McHenry, Illinois 31 joins Illinois 120 west for a short distance:
Still in McHenry, Illinois 31 splits from Illinois 120, which continues west: