In the U.S. state of Montana, Interstate 15 continues onward from Idaho for nearly 400 miles (640 km) through the cities of Butte, Helena, and Great Falls, intersecting with I-90, I-115, and I-315. I-15 reaches its northern terminus at the international border with Alberta, Canada.

Alberta Provincial Highway No. 4, commonly referred to as Highway 4, is a 103-kilometre (64 mi) highway in southern Alberta, Canada that connects Highway 3 in Lethbridge to Interstate 15 in Montana. The highway was designated in 1999 as the First Special Service Force Memorial Highway in honour of elite soldiers who travelled to Helena, Montana for training before World War II. The highway continues into the United States retaining that name.