US Route 666 or the 6th spur of US Route 66. It was established along with the rest of the US Route system in 1926. US Route 666 (as US Route 491 does now) came directly off US Route 66 in Gallup, NM and headed north mostly through Navajo Reservation land into Colorado, and Utah.
Nowadays, US Route 666 is now US Route 491. The Route was switched from 666 to 491 in 2003. Many believe that this switch was at the hands of religious officials and church groups lobbying to get rid of the "Mark of the Beast". The reality is it was actually something far more mundane, and downright boring that bought the switch. States with 666 running through them finally made the switch do to the fact that theft of Route 666 road signs where out of control.
The theft of these signs and infamy of Route 666 are all spawned by popular cultures images of the route. The name Route 666 was used for an Iron Maiden album. But the route has also been used as a movie title, and in television shows all with evil or paranormal connotations. The route has also been featured in a number of books fiction and non-fiction all with a similar mythos of evil and mystery.
These pop culture attributes associated with Route 666 have also lead many to believe that part of the route is evil due to its high mortality rates through accidents. Something many believe has dropped since the new numbering. In actuality the new numbering accompanied construction projects to make part of the route safer. But, to begin with though the route actually had relatively low mortality rates even as Route 666.
As for being evil though the Navajo, who had Route 666 travel through their land never saw anything evil about it. Part of the reason is that in Navajo beliefs 666 means nothing at all. The other part of it is that Navajos traveled the route regularly and never encountered anything evil with the route. Considering Route 666 was a main route through Navajo land, driving on it was as ordinary for many of them as driving through the main drags of our home towns is for us. As a matter of fact the route is often referenced in Tony Hillerman novels as a common route his Navajo police characters take between towns.
Overall, once you get past the triple 6, there doesn't seem to be anything all that mysterious about the route. It serves as a part of the mystic of Route 66, the aura of mystery and paranormal characters we see elsewhere on the route somewhat concentrated in this small little section that spurs off from it. It's easy to see as in many other parts of Route 66, that the mystery of it all depends on weather you live there and what part the route plays in your life. For a Chicagoan, or Los Anglian the triple 666 conjured up evil, but the fir the Navajo who lived, and worked off the route it was as normal as Main Street.