U.S. Route 191 is a spur of U.S. Route 91 that has two segments. The southern segment runs for 1,102 miles (1,773 km) from Douglas, Arizona on the Mexican border to the southern part of Yellowstone National Park. The northern segment runs for 442 miles (711 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 segments. The highway passes through the states of Arizona, Utah, Wyoming, and Montana.

U.S. Route 91 or U.S. Highway 91 is a 172.7-mile-long (277.9 km) north–south United States highway running from Brigham City, Utah, to Idaho Falls, Idaho, in the U.S. states of Idaho and Utah. Despite the "1" as the last digit in the number, US 91 is no longer a cross-country artery, as it has mostly been replaced by Interstate 15. The highway currently serves to connect the communities of the Cache Valley to I-15 and beyond. Prior to the mid-1970s, US 91 was an international commerce route from Long Beach, California, to the Canada–US border north of Sweetgrass, Montana. US 91 was routed on the main streets of most of the communities it served, including Las Vegas Boulevard in Las Vegas and State Street in Salt Lake City. From Los Angeles to Salt Lake, the route was built along the corridor of the Arrowhead Trail. A portion of the highway's former route in California is currently State Route 91.