The American Association of State Highway Officials (AASHO) approved the United States Numbered Highway System on November 11, 1926, and the MSHD designated US 31 in Michigan according to AASHO's plan to run northward from the Indiana state line and along the Lake Michigan shoreline to Mackinaw City. A section in the Benton Harbor–St. Joseph area overlapped US 12 and the modern concurrency with US 10 was also in place. By the end of the next year, the highway was extended across the Straits of Mackinac on the state car ferries to connect to US 2 in the Upper Peninsula north of St. Ignace. In 1930, Muskegon was bypassed; the new highway east of downtown was numbered US 31A.