1992. By the way, the "666" designation is simply the fact that it was the 6th branch off US Route 66. Added in 1926.
In 1992, Arizona requested a new number for its portion of U.S. Route 666, arguing the road signs on this highway were the most frequently stolen in the state. As a result, US 191 was extended again to the Mexican border at Douglas, Arizona. In 1999 it became the latest U.S. highway to run from border to border, with the extension from Malta, Montana to the Canadian border, absorbing former Montana Secondary Highway 242. In 2003 New Mexico asked AASHTO to renumber its portion of US 666. This time all of then US 666 was renumbered U.S. Route 491, the x91 number was decided because the road meets US 191 in Monticello, Utah.
Over the years, U.S. 666 has sometimes been the object of controversy because "666" is the "number of the beast" (or Antichrist) in the Bible. Revelation 13:18 states:
Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666.
This association is a result of gematria, a system of numerology that assigns a value to each letter of the alphabet. A word can, thus, be counted by adding the value of each letter. Through gematria, "666" is the biblical "number of the beast" because the letters comprising the name of the "beast" add up to 666. The identity of the "beast" is unclear, although the Roman Emperor Nero is a possibility.
The association with the "beast" earned U.S. 666 the nickname "Devil's Highway." USA Today quoted a State trooper who recalled one drunken-driving suspect on U.S. 666 who told him, "Triple 6 is evil. Everyone dies on that highway" (August 4, 1990). The Wall Street Journal titled an article "Beast of a Highway: Does Asphalt Stretch Have Biblical Curse?" (August 3, 1995). Referring to the highway's dangers, the article quoted a resident who "blames Satan. After all, 'the highway has the devil's name.'" It was also the subject of a cartoon in The New Yorker's issue of February 23/March 2, 1998 (a Corvette-type open top sports car is passing the U.S. 666 sign; the driver and his passenger are depicted as satanic figures).
In 2001, Lions Gate Home Entertainment released "Route 666," a movie staring Lou Diamond Phillips. He and Lori Petty play FBI agents assigned to deliver a government informant to court. The Internet Movie Data Base summarized the plot:
Smith, a mob informer hiding out with the Witness Protection Program, decides to make a break for it and hide out in the Arizona desert. The Feds catch up with him and rescue him just before a group of hitmen can manage to silence him for good. In the course of getting Smith away from the mafia thugs, the pair of agents assigned to protect him turn onto an abandoned stretch of highway nicknamed "Route 666" after the mysterious death of a prison chain gang. As the three continue on their way, they soon discover just what happened to the chain gang, and how the highway earned its name.
The route also played a small part in director Oliver Stone's controversial 1994 movie Natural Born Killers.
Despite the biblical reference and the image U.S. 666 has gained over the years, the gematria calculation had nothing to do with the numbering of the route. Boring though it may be to contemplate, the route was simply the sixth branch of U.S. 66 in early August 1926-and retained that number when U.S. 466 was dropped a few weeks later.
I live in SE Arizona and travel Route 191 frequently.