<Li> United States Numbered Highway System </Li> <Ul> <Li> List </Li> <Li> Special </Li> <Li> Divided </Li> <Li> Replaced </Li> </Ul> <P> U.S. Route 31 or U.S. Highway 31 (US 31) is a major north--south highway connecting 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 . </P> <P> 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 . </P>

Where does us highway 31 start and end
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