Entering Montana on its western border near a town called Troy, the Montana section of U.S. Highway 2 starts its number at zero. It meanders across the top protion of Montana, skirting under Glacier National Park as it continues eastward. It is the main corridor across the top of the state and racks up 667 miles before transitioning to the North Dakota section of U.S. Highway 2 and resetting the number back to zero. This is one of the highest numbered sections of road in the country except for I-10 in Texas. (not sure about Highway 101 in California).
This is not the longest highway in Montana but it has the highest mile marker. Highway 200 (706 miles), keeps getting knocked back to 0 each time it comes to a major intersection, leaving the state near Fairview at a paltry 66 miles.