The northernmost of the three monumental bridges along I-75 is the International Bridge, linking the twin cities of Sault Ste. Marie in Michigan and Ontario. The governments on each side of the international border formed a bridge authority to build a highway bridge in 1935. Construction started on the structure September 16, 1960. The International Bridge is nearly long, encompassing spans over the American and Canadian navigation channels for the Soo Locks and the St. Mary's River. The American approach is , and the Canadian approach is . The center span over the river is , flanked by and spans over the American and Canadian shipping channels, respectively. The bridge was designed by the same firm that handled the Mackinac Bridge for a cost of $20 million (equivalent to $ in ). It opened to traffic on October 31, 1962.