The highway that became U.S. Route 66 was initially proposed as U.S. Route 60.
During the planning of the United States Numbered Highway System in the 1920s, promoters wanted a major east–west route linking Chicago with Los Angeles. The number 60 was considered for this road, but another planned highway was assigned U.S. Route 60 instead. The Chicago–Los Angeles highway therefore received the number 66.
The choice of 66 was practical as well as memorable: the even number identified an east–west route, and the repeated digits made it easy to recognize. U.S. Route 66 was officially established in 1926 and became one of the best-known highways in American history.
A common mix-up is assuming that Route 66 was numbered first and later renamed. In fact, the proposed U.S. Route 60 designation came before the final number was selected. The road was later removed from the federal highway system in 1985, although many original segments remain preserved or signed as historic Route 66.