 US Highway 25 ( US 25 ) was a part of the United States Numbered Highway System in the state of Michigan that ran from the Ohio state line near Toledo and ended at the tip of The Thumb in Port Austin . The general routing of this state trunkline highway took it northeasterly from the state line through Monroe and Detroit to Port Huron .
 hopefully by the end of the year. The roadway was scheduled in 1919 from the state line through Monroe to Detroit and then to Port Huron . The road was extended to the state line in 1926, and in 1936 it was extended