By 1970, US 666 had reached its maximum historical extent, from Douglas AZ to Monticello UT (represented by the blue line on this map):
AASHTO denied the request (interestingly, that was the same meeting during which AASHTO approved the elimination of the last remaining segment of US 66, which had been the "parent" route after which US 666 was numbered). Then in 1987 Utah submitted an almost-identical proposal. The only difference was that the far northwestern segment (shown in green) would have connected with I-70 at Sigurd (instead of Richfield). That request was also denied. The US 666 designation no longer exists; it was replaced by US 491 in 2003 (more about the history of US 666 here.)
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