Northwest Illinois

As the highway enters Illinois across the Julien Dubuque Bridge, it continues southeasterly for a while, paralleling the Upper Mississippi River before turning east into the rugged country of the Driftless Area in Jo Daviess County, then turning flat and straight west of Stockton and entering Stephenson County. The Ulysses S. Grant Home, a national landmark, fronts near the highway at the eastern edge of Galena.

The road between Dubuque and Stockton was once known as the most dangerous stretch of road because of the hills and curves flanked by cliffs and valleys. Travelers were greeted with signs reminding them to drive carefully as they entered this stretch of road.

In Freeport, US 20 is routed onto a controlled-access freeway bypass. Between Freeport and Rockford, the route is a four-lane divided highway with two traffic light-controlled intersections, at Pecatonica Road and at Winnebago Road/Elida Street. The Rockford bypass routes US 20 onto a full controlled-access freeway; it remains this way until it overlaps Interstate 39 (I-39) southeast of downtown Rockford.

Rockford to Elgin

East of Rockford, the routing of US 20 runs parallel with I-90; US 20 serves minor cities that are not accessible from the Jane Addams Memorial Tollway. It is a four-lane divided highway from I-39 to just east of Belvidere; for the rest of this segment, it is a winding, rural two-lane road.

US 20 travels through the downtowns of the small towns and unincorporated areas of Garden Prairie, Harmony and Starks, as well as the city of Marengo. At Starks, eastbound US 20 overlaps southbound Illinois Route 47 (IL 47) and westbound IL 72, forming a wrong-way concurrency. All three routes pass beneath a railroad viaduct before separating south of Starks.

Continuing east, US 20 travels through Pingree Grove before entering the Elgin area. Before entering Elgin, US 20 widens to a four-lane controlled-access freeway that bypasses Elgin, around the town's south end. This road is known as the Elgin Bypass.

Elgin to Indiana

At the end of the Elgin bypass, US 20 takes on the name of Lake Street. US 20 has numerous high-volume intersections, including a grade-separated interchange with IL 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 (1.6 km) until the Elgin–O'Hare Expressway.

US 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 I-290 and onward to IL 53 before it returns to four lanes while serving downtown Addison, paralleling I-290 to the north. Past Elmhurst, the road overlaps I-290 for a mile (1.6 km).

US 20 continues through Stone Park and Melrose Park before turning south and joining US 12/US 45 on Mannheim Road. In La Grange, the combined US 12, US 20, and US 45 traveling on La Grange Road intersect with Ogden Avenue, which carries US 34, one of the few places where four U.S. Numbered Highways intersect. La Grange Road continues south until US 20 turns onto 95th Street eastbound with US 12. This combination later joins with US 41, where the three routes follow Ewing Avenue and Indianapolis Boulevard, which are the last portions of US 20 prior to its crossing the state line into Indiana.

Elgin Bypass

Construction of the Golf Road (IL 58) extension to Summit Street in Elgin was completed in 1932. In 1958, I-90 was built through the north side of Elgin extending east to Devon Avenue and on to Chicago by 1960. In 1935, concerned about increasing numbers of traffic, residents of Elgin laid plans to build a bypass. US 20 had been routed down Villa Street, Highland Avenue, and Larkin Avenue through downtown Elgin. A section of freeway opened south of Elgin as the Elgin Bypass 27 years later.^[5]

The segment of the Elgin Bypass between McLean Boulevard and Grace Street (IL 25), including the Fox River bridge, was opened first. Later, the outer segments of the bypass—to Randall Road on the west and Bluff City Boulevard on the east—were opened. On November 30, 1962, Governor Otto Kerner Jr. dedicated the Elgin Bypass.^[6]

From 1962 until 1984, the old route was designated as US 20 Business (US 20 Bus.).^[3]

In 2012–2016, the interchange between the Elgin Bypass and McLean Boulevard was upgraded to a single-point urban interchange, the bridge was widened and replaced, and portions of the bypass were repaved at a cost of $44.4 million (equivalent to $55.3 million in 2023^[7]).^[8][9]

In 2019, the interchange between the Elgin Bypass and Randall Road received a $9-million (equivalent to $10.6 million in 2023^[7]) upgrade so that northbound Randall Road traffic can enter the eastbound Elgin Bypass without making a left turn at a traffic light.^[10]

Starting in 2014, a comprehensive study of the Elgin Bypass corridor resulted in a number of recommendations including a wider bridge across the Fox River, a replacement bridge for IL 31 crossing the bypass, and replacement of other bridges. The 2019 estimates of the recommended improvements total $84 million (equivalent to $98.7 million in 2023^[7]), with $10.6 million (equivalent to $12.5 million in 2023^[7]) designated for the IL 31 bridge.^[11]