U.S. Route 20 in Iowa

US 20 enters Iowa on the Sergeant Floyd Memorial Bridge, which also carries I-129 and US 75, over the Missouri River. Almost immediately upon landing in Iowa, there is an interchange with I-29, at which I-129 ends. Continuing east, US 20 and US 75 run together around the southern and eastern sides of Sioux City. The two routes split at the Gordan Drive (US 20 Business, or US 20 Bus.) interchange as US 20 exits to the east from Sioux City. The expressway is an older section that goes through Lawton and comes to a stop sign at Moville. About three miles (4.8 km) further east, at Ida Avenue (County Route D22, or CR D22), the newest four-lane stretch (except for a short expressway segment just south of Holstein that is briefly concurrent with US 59)[2] starts and goes to about one mile (1.6 km) west of an interchange with US 71 and Iowa Highway 471 (Iowa 471), just north of Early.[3] From this point until an interchange with Iowa 4 northwest of Rockwell City is the second newest section of four-lane, which opened in November 2012.[4] This new roadway passes to the north of Sac City and Lytton and has another interchange with the realigned route of US 71 northeast of Sac City but mostly has at-grade intersections, including CR M54 into Sac City and CR N28 into Lytton.

East of Iowa 4, and extending to the former beginning of the four-lane expressway northeast of Moorland (now an interchange with CR D36), is a four-lane section which opened in December 2010 and bypasses Rockwell City to the north. US 20 continues until just south of Highview, when it becomes a full Interstate-standard freeway. The expressway south of Fort Dodge has interchanges with US 169 and CR P59. East of Fort Dodge, US 20 overlaps with Iowa 17 for 4.5 miles (7.2 km), ending at Webster City. Near Williams, US 20 intersects I-35. Beginning at the interchange with US 65 is a section of four-lane freeway which, when completed, shaved nearly 16 miles (26 km) off US 20's length. Prior to 2003, US 20 joined US 65 north through Iowa Falls and then east toward Ackley and Parkersburg before heading south again to the freeway with Iowa 14. The bridge crossing the Iowa River near Steamboat Rock is a launched steel I-girder bridge, the first of its kind in the U.S.[5] The bridge was designed to cross the old-growth woodland of the Iowa River valley with minimal impact.

US 20 continues east toward Cedar Falls where it overlaps Iowa 58 for just over one mile (1.6 km). The western junction of Iowa 58 (Hudson Road) connects traffic to the University of Northern Iowa while the eastern junction of Iowa 58 is also the western junction of Iowa 27, the Avenue of the Saints. Another mile (1.6 km) east, in Waterloo, US 20 intersects US 63 (Sergeant Road). On the southeast side of Waterloo, I-380 joins US 20/Iowa 27 for six miles (9.7 km) before exiting with Iowa 27 south toward Cedar Rapids.

US 20 continues as a freeway east of Waterloo. It intersects Iowa 150 at Independence and Iowa 13 at Manchester, as well as junctioning a number of county roads serving smaller communities. Approximately 21 miles (34 km) between Independence and Manchester, the highway goes over a terminal moraine and enters the Driftless Area, a region it will not exit until reaching Stockton, Illinois.

U 20 crosses the Mississippi River at Dubuque using the Julien Dubuque Bridge.

At Delaware, US 20 changes from a freeway to an expressway with at-grade intersections; however, there are many interchanges where US 20 meets major roads, mostly at the villages and towns along US 20's route. US 20 serves Earlville, Dyersville (where US 52 joins the route and passing near the Field of Dreams filming site), Farley, Epworth, and Peosta before entering Dubuque. On the western edge of Dubuque, an interchange onto the Southwest Arterial routes US 52 around the southern outskirts of the city, connecting to US 61 and US 151. US 20 remains at least four lanes through Dubuque, but there are nine stoplights along the route, particularly on the western edge of the city. There is a brief controlled-access section where the highway descends into the Mississippi River valley. Near the river, Locust Street (Iowa 946) connects US 20 with US 61 and US 151, which have entered the city from the south as an expressway and continue northward as a freeway. US 20 crosses from Iowa into Illinois via the Julien Dubuque Bridge.

US 20 was designated in Iowa on October 16, 1926. The designation created a single route number for drivers to follow across the state from Sioux City to Dubuque; previously, three route numbers were used.

It is possible to drive most of US 20's former two-lane alignment from Early to Dubuque. Most of this routing (with the exception of Iowa 57 from US 65 to Cedar Falls) is now under city and county control, but most of the guide signs remain from when US 20 followed this route. From Moorland, the highway's former alignment is US 20 Bus. through Fort Dodge. At CR P59, it continues on CR D20. The road changes designations several times between this point and Iowa Falls, but its route is very clear. It follows US 65 north out of Iowa Falls to the junction of Iowa 57, then follows Iowa 57 to Cedar Falls. At Cedar Falls, the former alignment continues on US 218 to Broadway Street; through Waterloo, it follows Broadway to Franklin Street to Dubuque Road. Beyond Waterloo, the designation of the road again changes several times, but the route to follow remains clear. Between Dyersville and Farley, US 20 follows its historic alignment. At Farley, the former alignment follows Old Highway Road into Dubuque.

Iowa Highway 946 (Iowa 946) is a short route that begins at a partial trumpet interchange along the US 61/US 151 expressway. Northbound traffic passes beneath the expressway and joins southbound traffic on Locust Street. In the half-mile (800 m) between US 61/US 151 and US 20, Iowa 946 passes a light industrial area containing a Hy-Vee grocery store.

Iowa 946 intersects US 20, known locally as Dodge Street, at a signal-controlled intersection. Immediately east of this intersection is the approach to the Julien Dubuque Bridge, which carries US 20 across the Mississippi River into East Dubuque, Illinois. Iowa 946 continues north on Locust Street for 1,000 feet (300 m) through another light industrial area. At the edge of downtown Dubuque, Iowa 946 turns east for 600 feet (180 m) where it meets US 61/US 151 just west of Dubuque's Ice Harbor. Iowa 946 was created along Locust Street upon the staggered completion of an expressway starting in 1991 which relocated US 52, US 61, and US 151 away from Locust Street and closer to the Mississippi River. The new expressway carried the three routes beneath the approach to the Julien Dubuque Bridge, making a direct intersection impossible. Since all four routes are U.S. Highways and come from all directions, it was imperative to keep the routes connected. With the completion of the Southwest Arterial around the southern edge of Dubuque, US 52 has been rerouted away from the downtown area.

• US 20 at the Iowa Highways Page
• U.S. 20 Corridor Association (group advocating completion of the four-lane expressway in western Iowa)