Llewellyn Toulmin has a story in The Sentinal Newspapers about a trip on Route 66, with most of the material taking place in Illinois and Missouri.
Then there’s this passage:
The history of Route 66 is fascinating but complicated. It was not envisioned as a major road at first, as witnessed by its number. In 1926, when the route numbering system was launched, major coast-to-coast roads were given numbers ending in zero, similar to our modern interstates. So 66 was not originally planned as a major highway.
Route 66 might have been considered minor by a few wags in the federal government when it was assigning highway numbers in the mid-1920s. But the highway’s patriarchs knew what they were doing. They knew a southern route from Chicago to Los Angeles that would naturally have fewer problems with winter weather would be a big deal to truckers and cross-country travelers.
In fact, Cyrus “Father of Route 66” Avery wanted to name the highway U.S. 60, thus giving it the “major” highway designation. But some folks in Kentucky also wanted U.S. 60 in their state. A long, dragged-out fight might have ensued until Avery suggested that U.S. 66 would be acceptable for his route.
And so U.S. 60 became just another east-west highway. U.S. 66 not only became a major route, but internationally famous as well.