States traversed: Montana, Wyoming, Utah, and Arizona
US Route 191 runs from Canada to Mexico in the Mountain West region of the United States. It is very useful to reaching many of America's national parks, though it doesn't connect to any major cities. Unusually, US Route 191 is much longer than its parent route, US Route 91, which was heavily truncated due to the advent of Interstate 15 replacing most of the route. The northern end of US Route 191 in north-central Montana, north of the town of Malta. In Wyoming, US Route 191 runs through Yellowstone and Grand Teton National Parks. In Utah, US Route 191 runs through Vernal, home of Dinosaur National Monument. Farther south in Utah, the highway is flanked by Arches National Park on the east side of the road, and by Canyonlands National Park on the west side. In Arizona, US Route 191 runs alongside Petrified Forest National Park before ending at the Mexican border, where Douglas, Arizona, meets Agua Prieta, Mexico.
My photo of US Route 191 shows a sign assembly indicating where it joins Interstate 70 west, just north of Moab and Arches National Park in east-central Utah. This photo was taken in late April 2017 after visiting Arches National Park, on my trip encircling the Grand Canyon to see Arches and four other national parks, plus Las Vegas. You can see from looking at this photo that US Route 6 runs concurrent with Interstate 70 here; what you can't see is that US Route 50 runs concurrent with it as well (meaning this sign assembly is incomplete).