For its entire length, US 20 is designated as the General Ulysses S. Grant Highway, often abbreviated the U.S. Grant Highway. However, this name is only commonly used west of Elgin. From Elgin to Melrose Park, US 20 is commonly called Lake Street. It then is called Mannheim Road, and further south La Grange Road. US 20 is then called 95th Street as it turns east through Chicago's southwestern suburbs. It also takes on the names of several smaller streets through the southern portion of Chicago, before leaving Illinois on Indianapolis Boulevard.
Approaching Freeport, U.S. 20 is routed onto a two-lane bypass around the city. The Rockford bypass routes U.S. 20 onto a full limited-access freeway; it remains this way until it overlaps Interstate 39 southeast of downtown Rockford.
U.S. 20 travels through the downtowns of the small towns and unincorporated areas of Garden Prairie, Marengo, Harmony and Starks. At Starks, eastbound U.S. 20 overlaps southbound Illinois Route 47 and westbound Illinois Route 72, forming a wrong-way concurrency. All three routes pass beneath a railroad viaduct before separating south of Starks.
Continuing east, U.S. 20 travels through Pingree Grove before entering the Elgin area. Before entering Elgin, U.S. 20 widens to a four lane limited-access freeway that bypasses Elgin. This road is known as the Elgin Bypass.
At the end of the Elgin bypass, U.S. 20 takes on the name of Lake Street. U.S. 20 has numerous high-volume intersections, including a grade-separated interchange with Illinois Route 59. In Hanover Park, an early-2000s reconstruction project widened lanes on the road and created a median for safer travel; east of Barrington Road / County Farm Road, the road briefly becomes six lanes for one mile until the Elgin-O'Hare Expressway.
U.S. 20 continues in an east-southeast direction through the town center of Bloomingdale and becomes six lanes at the Bloomingdale-Roselle border. It maintains six lanes through a very high traffic corridor to Interstate 290 and onwards to Illinois Route 53 before it returns to four lanes while serving downtown Addison, paralleling Interstate 290 to the north. Past Elmhurst, the road overlaps Interstate 290 for a mile, technically giving U.S. 20 ten lanes of traffic, although the rightmost two are exit-only lanes. At the interchange, westbound U.S. 20 has only one lane, but eastbound U.S. 20 has two.
U.S. 20 continues through Stone Park and Melrose Park before turning south and becoming the concurrent U.S. Routes 12/20/45. At this point, U.S. 20 becomes Mannheim Rd, and later, La Grange Road, until it turns onto 95th Street eastbound with U.S. 12. This combination later joins with U.S. Route 41, where the three routes follow Ewing Avenue and Indianapolis Boulevard out of the state.
U.S. 20 has been rerouted around the cities of Freeport, Rockford, Elgin and Chicago. The bypass around Freeport is currently only two lanes wide; future construction will widen this segment to four lanes. A bypass is currently planned for Galena; see Future.
The longest Business U.S. Route 20 in Illinois runs 21 miles from just west of Rockford to just east of Belvidere. The route begins just west of Weldon Road and runs as State Street into downtown Rockford, where it is split into two one-way streets (Chestnut Street and Walnut Street for eastbound traffic, Jefferson Street for westbound traffic). The route continues east along State Street all the way through Rockford and becomes a two-lane highway between Rockford and Belvidere. Business 20 is then routed in an outer loop around the city of Belvidere before merging back with US 20 near Genoa Road and Interstate 90 (Northwest Tollway).
Business U.S. Route 20 in Elgin was removed in 1984. This ran along Larkin Avenue, Highland Avenue, Chicago Street and Villa Avenue. The current routing of U.S. 20 south of Elgin is called the Elgin Bypass.