U.S. Route 191 is a spur of U.S. Route 91 that has two branches. The southern branch runs for from Douglas, Arizona on the Mexican border to the southern part of Yellowstone National Park. The northern branch runs for from the northern part of Yellowstone National Park to Loring, Montana, at the Canadian border. Unnumbered roads within Yellowstone National Park connect the two branches. The highway passes through the states of Arizona, Utah, Wyoming, and Montana.