U.S. Route 12 (US 12) in the U.S. state of Illinois is an arterial highway that runs northwest to southeast through the Chicago metropolitan area.

It continues to the Fox Lake area, at which point it becomes a four-lane divided highway, with occasional interchanges.

From the state line to this point, US 12 is also marked as the main route for the Fox River Valley, which continues south on IL 59.

In downtown Des Plaines, US 12/45 has a major intersection with US 14 (Miner Street) at a busy at-grade crossing with the Harvard Subdivision of the Union Pacific Railroad.

US 12/US 45 runs through various streets in Des Plaines before following Mannheim Road north of O'Hare International Airport.

At the southeast corner of the airport, a signalized intersection terminates a brief expressway portion of IL 19 (Irving Park Road).

In Melrose Park, US 12/US 45 has a grade-separated intersection with IL 64 (North Avenue), a major east–west artery west of Chicago.

In downtown La Grange, US 12/US 20/US 45 intersects with US 34 (Ogden Avenue), making it the only location in the Chicago area (and possibly the state of Illinois) where four U.S.

[citation needed] In addition, there is a level-grade crossing with BNSF Railway tracks very near the intersection, which leads to frequent traffic backups and delays.

It then crosses the Des Plaines River and Chicago Sanitary and Ship Canal before a complex interchange with IL 171 (Archer Avenue).

Just south of the Tri-State Tollway, US 12/US 20/US 45 becomes southbound 96th Avenue, passing through the Forest Preserve District of Cook County for nearly two miles (3.2 km) before US 12/US 20 split from US 45 onto eastbound 95th Street.

The Illinois portion of US 12/US 20/US 41 terminates beneath the I-90 bridge over Indianapolis Boulevard; this is also where the Chicago Skyway becomes the Indiana Toll Road.

From 1928 to 1929, the section between the Wisconsin state line to Crystal Lake was being constructed; this segment was designated Temporary U.S. Route 12 (Temp.

[7][8] The original pothole-full 1950s asphalt pavement with double yellow lines visible in the west bound lanes between Old McHenry and Old Rand roads has since been resurfaced in 2013.

[9][10] Mature trees lining the frontage roads and 1950s-style motels and diners remain in Fox Lake.