U.S. Route 191 (US 191) is a spur of U.S. Route 91 that has two branches. The southern branch runs for 1,465 miles (2,358 km) from Douglas, Arizona on the Mexican border to the southern part of Yellowstone National Park. The northern branch runs for 440 miles (710 km) from the northern part of Yellowstone National Park to Loring, Montana, at the Canada–US border. Unnumbered roads within Yellowstone National Park connect the two branches. The highway passes through the states of Arizona, Utah, Wyoming, and Montana. The highway was designated in 1926 and its routing has changed drastically through the years. The modern US 191 bears almost no resemblance to the original route, which was primarily in the state of Idaho. Most of the current route of US 191 was formed in 1981. Since the extensions in the 1980s and 1990s, U.S. Route 191 is much longer than its parent route to which it no longer connects, and it is one of the longest U.S. three-digit routes.
