Illinois Route 173 (IL 173) is a 66.20-mile-long (106.54 km) east–west state highway that runs from Illinois Route 251 in the Rockford suburb of Machesney Park east to Illinois Route 137 (Sheridan Road) in Zion near the Illinois-Wisconsin border. Illinois Route 173 is also one of the northernmost east-west state highways in Illinois. On Interstate 94 (Tri-State Tollway) traveling south from Wisconsin, it is the second road seen which has an interchange at that location. Similarly, when traveling south/east from Wisconsin on Interstates 39/90 and U.S. Route 51 (Jane Addams Memorial Tollway), Illinois Route 173 is only the third exit into Illinois. In 2008, the entirety of Illinois 173 has been designated the 173rd Airborne Brigade Highway

Illinois 173 travels across relatively hilly northern Illinois from the Rockford area to Zion. This area is more densely populated than other portions of rural Illinois. It is two lanes for over 90% of its length.

In July 2007, a new interchange was constructed at Illinois 173, Interstates 39/90, and U.S. Route 51 (Jane Addams Memorial Tollway).

SBI Route 173 originally ran from near downtown Rockford (along Forest Hills Road) to Zion. During the 1980s, when the village of Machesney Park was heavily developed and incorporated, Illinois Route 173 was scaled back and its western terminus moved to Illinois Route 251 (North 2nd Street). From 1972 to 1994 the highway also assumed a portion of what is now Illinois Route 137 north to the Wisconsin border.

County	Location	mi	km	Destinations	Notes
Winnebago	Machesney Park	0.0	0.0	IL 251 (2nd Street)
	Machesney Park–Loves Park village line	1.3	2.1	CR 11 (Perryville Road)
		3.6	5.8	I-39 Toll / I-90 Toll / US 51 Toll (Jane Addams Memorial Tollway) – Chicago, Rockford, Madison
	Caledonia	4.6	7.4	CR 25 (Belvidere Road)
Boone		9.4	15.1	IL 76 (Fairgrounds Road)
McHenry	Harvard	22.8	36.7	US 14 east (Division Street)	West end of US 14 overlap
		23.3	37.5	US 14 west (Division Street)	East end of US 14 overlap
		25.4	40.9	CR A18 east (Altenberg Road)
	Alden	28.9	46.5	CR A15 west (Oak Grove Road)
	Hebron	34.1	54.9	IL 47 (Main Street)
	Richmond	39.1	62.9	CR V30 (Keystone Road)
		40.6	65.3	US 12 (Main Street)
	Spring Grove	44.5	71.6	CR V43 south (Richardson Road)
Lake	Antioch	48.5	78.1	CR V51 north (Lake Avenue)
		51.3	82.6	IL 59 south
		52.1	83.8	IL 83 (Main Street)
		56.1	90.3	US 45
	Old Mill Creek	58.3	93.8	CR W15 (Hunt Club Road)
		58.8	94.6	I-94 Toll east (Tri-State Tollway) – Chicago
	Rosecrans	59.7	96.1	US 41 (Skokie Highway) to I-41 north / I-94 west
	Zion	63.0	101.4	IL 131 (Green Bay Road)
		64.4	103.6	CR W34 (Lewis Avenue)
		66.20	106.54	IL 137 / LMCT (Sheridan Road)
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus

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