 M @-@ 122 was initially assumed into the state highway system in 1929 as a connector between US 31 and Straits State Park . In 1936 , US 2 was routed into St. Ignace and US 31 was scaled back to end in the Lower Peninsula in Mackinaw City . M @-@ 122 now provided a connection between US 2 and the new docks on the southeast side of the city . It existed in this capacity until 1957 when the Mackinac Bridge opened to traffic . 
