US Highway 12 (US 12) is an east–west US Highway that runs from Aberdeen, Washington, to Detroit, Michigan. In Michigan it runs for 210 miles (338 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 the southeastern. In between Coldwater and the Ann Arbor area, the highway angles northeasterly and passes the 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. 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 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 112S for a few years in the 1930s.
Between the state line near Michiana and the interchange with I-94 near New Buffalo, US 12 forms a portion of the Lake Michigan Circle Tour (LMCT). The full length of the highway in the state is also a Pure Michigan Historic Byway. Outside of the various cities, most of US 12 is a rural, two-lane state highway. One section runs concurrently with I-94 south of Ypsilanti. From there eastward, the US 12 is a divided highway and then a boulevard into the Detroit area. The entire length of the highway east of Coldwater is listed on the National Highway System, a network of roads important to the US's economy, defense, and mobility.
US 12 enters the state of Michigan southwest of New Buffalo near the town of Michiana. The highway runs a bit inland and parallel to the Lake Michigan shore on Red Arrow Highway past the community of Grand Beach before turning eastward away from the lake on the north side of New Buffalo. US 12 intersects I-94 less than a mile east of this turn and continues due east along Pulaski Highway through rural farmland. The highway is the main east–west street as it crosses through Three Oaks in southern Berrien County. East of Galien, the highway dips southward around Dayton Lake before passing south of Buchanan. The highway continues eastward, intersecting US 31 (St. Joseph Valley Parkway) near Niles. East of that freeway, US 12 expands into an expressway as the Niles Bypass. This bypass intersects M-139, crosses the St. Joseph River and intersects M-51 on the south side of Niles. US 12 turns northeasterly along the bypass east of downtown and then departs to the southeast at an interchange with Main Street and M-60 just across the county line with Cass County.
Continuing through southern Cass County, US 12 runs roughly parallel to the state line. It turns northeasterly to run into Edwardsburg, where it intersects M-62. The highway turns southeasterly and runs to the south of Eagle Lake before entering the community of Adamsville. The highway continues on this southeasterly course until it intersects the former M-205 and M-217 (Michiana Parkway) near Union and turns to the northeast. US 12 intersects the southern end of M-40 before crossing into St. Joseph County at the eastern crossing of the St. Joseph River.