Devil’s Highway is known as one of the twistiest roads in the US by motorcycle riders. Even the Federal Highway Administration considers this drive to be the curviest road in the nation.
U.S. 191 (Old 666) is a pretty long highway. You can find it running through Montana, Wyoming, Utah and Arizona. The very twisty Devil bits are located in Arizona, this section is also known as the Coronado Trail Scenic Byway, which runs North - South along the New Mexico border. The Coronado Trail follows the route near Coronado's path as he searched for the "Seven Cities of Cibola" over 450 years ago. 
In the US there has always been a lot of superstition around places with numbers like 13 or 666. Many tall building don’t even have a 13th floor since 13 is considered an unlucky number. The number 666 has been associated as the number of the beast, or the antichrist so Arizona changed the numbering from 666 to U.S. 191 in 1993. The other states also changed their numbering too.
U.S. Route 191 is a major north–south state highway through the eastern part of the U.S. state of Utah. The present alignment of US-191, which stretches from Mexico to Canada, was created in 1981 through Utah. Previously the route had entered northern Utah, ending at US-91 in Brigham City, but with the completion of I-15 it was truncated to Yellowstone National Park and re-extended on a completely different alignment. In addition to a large portion of US-163, this extension absorbed several state routes: SR-33, most of SR-44, and SR-260.
