The former US 666 was the only highway to have passed through each of the Four Corners states, even though it never came near the Four Corners Monument, which is accessed via US 160. The Arizona portion was renumbered separately and is now part of US 191. US 491 serves the states of New Mexico, Colorado and Utah. Other features along the route include Mesa Verde National Park and Dove Creek, Colorado, the self-proclaimed pinto-bean capital of the world. The highway passes by two mountains considered sacred by Native Americans: Ute Mountain and an extinct volcanic core named Shiprock. The highway, now a spur route of US 91 via its connection to US 191, runs through New Mexico, Colorado and Utah, as well as the tribal nations of the Navajo Nation and Ute Mountain Ute Tribe. There have been safety improvement projects in recent years, and fatality rates have subsequently decreased. These factors led to two efforts to renumber the highway, first by officials in Arizona, then by those in New Mexico. The problem was compounded by persistent sign theft. This Satanic connotation, combined with a high fatality rate along the New Mexico portion, convinced some people the highway was cursed.

With the US 666 designation, the road was nicknamed the 'Devil's Highway' because of the significance of the number 666 to many Christian denominations as the Number of the Beast. It was created in 2003 as a renumbering of U.S. Highway serving the Four Corners region of the United States. Route 491 ( US 491) is a north–south U.S. Map of the Four Corners area with US 491 highlighted in red

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