US Highway 12 (US 12) is an east–west United States Numbered Highway that runs from Aberdeen, Washington, to Detroit, Michigan. In Michigan, it runs for 210 miles (340 km) between New Buffalo and Detroit as a state trunkline highway and Pure Michigan Byway. On its western end, the highway is mostly a two-lane road that runs through the southern tier of counties roughly parallel to the Indiana state line. It forms part of the Niles Bypass, a four-lane expressway south of Niles in the southwestern part of the state, and it runs concurrently with the Interstate 94 (I-94) freeway around the south side of Ypsilanti in southeastern Michigan. In between Coldwater and the Ann Arbor area, the highway angles northeasterly and passes Michigan International Speedway. East of Ypsilanti, US 12 follows a divided highway routing on Michigan Avenue into Detroit, where it terminates at an intersection with Cass Avenue.
When US 12 was designated in Michigan on November 11, 1926, along with the other original US Highways, it ran along a more northerly course. It originally replaced sections of the original M-11 and M-17 along Michigan Avenue in the state, the route of the much older St. Joseph Trail, a footpath used by Native Americans before European settlement in the area. It entered from Indiana as it does now, but it followed the Lake Michigan shoreline farther north to Benton Harbor–St. Joseph before turning eastward to run through Kalamazoo, Battle Creek, and Jackson. In the Ann Arbor area, it followed a more northerly path into Detroit before terminating downtown. In the 1940s and 1950s, sections of the highway were converted into expressways and freeways. Starting in 1959, these freeway segments were renumbered as part of I-94, and, in January 1962, US 12 was shifted to replace US Highway 112 (US 112). That highway, when it was designated in 1926 replaced the original M-23 along the Chicago Road, which was the route of the older Sauk Trail. Later, US 112 replaced the first M-151 when the former was extended to New Buffalo in the mid-1930s. Since 1962, the highway has remained relatively unchanged aside from minor truncations in the city of Detroit. US 112 previously had two business loops, both of which were renumbered Business US Highway 12 (Bus. US 12) in 1962. In 2010, the Niles business loop was decommissioned, but the one in Ypsilanti remains. One section of the former US 112 was renumbered US Highway 112S (US 112S) for a few years in the 1930s.
