US 20 was part of the Dunes Highway from the Illinois state line to Michigan City, concurrent with US 12.

Only the segment of US 20 that is concurrent with the St. Joseph Valley Parkway is included as a part of the National Highway System (NHS).

[2] The NHS is a network of highways that are identified as being most important for the economy, mobility, and defense of the nation.

This measurement is a calculation of the traffic level along a segment of roadway for any average day of the year.

The road passes by both commercial and industrial areas, between Wolf Lake and Horseshoe Casino.

US 20 turns left onto Michigan Street and heads east to Gary passing both industrial and residential areas.

East of Downtown Gary, the eastbound and westbound lanes merge to form the Dunes Highway.

Before reaching the neighborhoods of northeastern Gary, the Dunes Highway has an interchange with I-65 and I-90/Indiana Toll Road.

[6] US 20 then meets SR 51 at the state road's northern terminus, near the boundary between Lake Station and Gary.

The road now enters Burns Harbor and has interchanges with I-94 and SR 49, just south of Indiana Dunes National Park.

On the east side of Michigan City, the route has an interchange with both SR 212 and I-94 followed by an intersection with the northern terminus of US 35.

Now concurrent with US 31, the St. Joseph Valley Parkway has another cloverleaf junction, with SR 2 on Western Avenue, before it curves to the southeast to run along the southwest side of South Bend.

The CR 17 interchange marks the eastern terminus of the limited-access freeway known as the St. Joseph Valley Parkway.

The road continues east into downtown Angola, passing just to the north of Trine University.

In rural eastern Steuben County, the route passes through farmland and has an intersection with the northern terminus of SR 1.

Narrower than anticipated, the new concrete highway was still superior to most Indiana roads, which, in the mid-1920s, were gravel or dirt with paved sections only between the larger towns.

[12][13] When US 20 was signed in Indiana, in 1926, the section from Illinois state line to Michigan City was concurrent with US 12 and the Dunes Highway.

[14][15] In the early 1930s, US 20 from the Illinois state line to Michigan City was moved to its current route.

When the state of Indiana built the St. Joseph Valley Parkway, the route downtown was decommissioned.