 US Highway 45 ( US 45 ) is a part of the United States Numbered Highway System that runs from Mobile , Alabama , to the Upper Peninsula ( UP ) of the US state of Michigan . The highway forms a part of the state trunkline highway system that is maintained by the Michigan Department of Transportation ( MDOT ) . It enters the state from Wisconsin south of Watersmeet , ending at an intersection with Ontonagon Street in Ontonagon . In between , the roadway crosses the UP running for approximately 54 3 ⁄ 4 miles ( 88 @.@ 1 km ) through the Ottawa National Forest and parallel to the Ontonagon River . 
