U.S. Route 31 or U.S. Highway 31 (US 31) is a major north–south United States highway connecting from southern Alabama to northern Michigan. Its southern terminus is at an intersection with US 90/US 98 in Spanish Fort, Alabama. Its northern terminus is at an interchange with Interstate 75 (I-75) south of Mackinaw City, Michigan.
US 31 once crossed the Straits of Mackinac by car ferry to intersect US 2 north of St. Ignace, Michigan, in the Upper Peninsula and then formerly reached Mackinaw City along the southern approaches of the Mackinac Bridge (which has been taken over by I-75). It also formerly entered downtown Mobile, Alabama, via a long bridge over Mobile Bay.
The southern segment of US 31 connects the major cities including Mobile, Montgomery, Birmingham, Huntsville in Alabama; and Nashville in Tennessee. The northern segment of US 31 connects Louisville in Kentucky; and Indianapolis in Indiana. Between Nashville and Louisville, US 31 splits to U.S. Route 31W and U.S. Route 31E. From the Mobile Bay area in Alabama to Indianapolis, US 31 travels largely parallel to Interstate 65 (I-65).
