US Hwy 666 started in Utah and ended in Arizona going through Colorado and New Mexico (the Four Corners)and the Navajo Nation. The stretch of road from Cortez, CO to Gallup, NM is a long 2-lane narrow road on which many people lost their lives due to accidents and alcohol. In 2003, after some improvements were made to the road, the number was changed.
http://www.geocities.com/usend6069/End666/end666.htm