High Quality Content by WIKIPEDIA articles! U.S. Route 31 is a long north–south highway connecting northern Michigan to southern Alabama, with its northern terminus at Interstate 75 near Mackinaw City, Michigan, and southern terminus at the combined U.S. Route 90 U.S. Route 98 at Spanish Fort, Alabama. This highway formerly reached Mackinaw City along the southern approaches of the Mackinac Bridge (which has been taken over by Interstate 75) in the north and downtown Mobile, Alabama in the south via a long bridge over Mobile Bay.