High Quality Content by WIKIPEDIA articles! US Highway 31 (US 31) is a part of the United States Numbered Highway System that runs from Alabama, to the Upper Peninsula of the US state of Michigan. In Michigan, it is a state trunkline highway that runs from the Indiana–Michigan state line at Bertrand Township to its terminus at Interstate 75 (I-75) south of Mackinaw City. The highway runs for 357.234 miles (574.912 km) in the state.