History: Mason County’s highways, US 31. By Rob Alway, Editor-in-Chief. Three highways run through Mason County: US 31, north and south, US 10, east and west, and M-116 north and south. Today’s article will discuss US 31. US 31 begins its northerly route in Spanish Fort, Alabama and ends in Mackinaw City. In Michigan, the highway runs from the Michigan-Indiana…