U.S. Route 31 (US 31) is a part of the United States Numbered Highway System that runs from Spanish Fort, Alabama, to Mackinaw City, Michigan.

North of SR 38, it reverts to an at-grade expressway until reaching the Kokomo area, where it again becomes a full freeway to bypass that city to the east.

There the route converges with US 20 (St. Joseph Valley Parkway) and proceeds west and then north to bypass South Bend.

After US 20 splits off to the west near the South Bend Airport, US 31 has an interchange with the Indiana Toll Road (I-80/I-90) before proceeding north into Michigan.

[1] In the late 2000s, INDOT began a process of converting US 31 to Interstate Highway standards from South Bend to Indianapolis.

[9][10][11][12] A portion of the Plymouth–South Bend section was dedicated as the Richard W. Mangus Memorial Highway, in honor of the local state representative who supported the freeway's construction, at the ribbon-cutting ceremony on August 27.

The remaining 78 miles (126 km) of US 31 between South Bend and Indianapolis is gradually being converted to a limited access highway without traffic signals, starting with an interchange at SR 28 in Tipton County which opened in November 2016.

[15] As of 2018, there were six traffic signals left between Indianapolis and South Bend, and a program introduced by Governor Eric Holcomb in September 2018 proposed to remove four of those.

However, after the state determined that the original plans would severely limit access within Tipton County, those upgrades were removed from consideration until further study could be completed and these issues mitigated.