Illinois Route 173 is the northernmost east-west state route in the eastern half of northern Illinois. It begins in Rockford's northern suburbs and heads east to end in Zion, a town along the shore of Lake Michigan. Toward the east, in Lake and McHenry Counties, Illinois Route 173 tends to be within a couple miles of the Wisconsin border. Farther west, in Boone and Winnebago Counties, the route nudges a bit farther south, to a latitude eight to nine miles south of the state line.
The western beginning of Illinois 173 is at Illinois 251 in Machesney Park, a northern suburb of Rockford:
Not until the last 15 years did Illinois 173 get an interchange with Interstate 90/39, but it does now:
Illinois 173 crosses Illinois 76, and it's the only state route to do so; this is near Poplar Grove. Take your pick between winter and summer:
In Harvard, Illinois 173 joins US Route 14 north for a short distance:
Still in Harvard, Illinois 173 splits off of US Route 14 to go east again:
As Illinois 173 is straightening out from an S-curve in Hebron, it crosses Illinois 47: