Which is a complete misnomer, as US numbered highways were and still are maintained by state and local governments [1].
I grew up near the "birthplace of Route 66" so yeah, I've heard of it. It was maintained and improved by a private organization (imagine that!) [2] until the federal interstate push of the 1950s.
[1] http://en.wikipedia.org/wiki/United_States_Numbered_Highways
[2] http://en.wikipedia.org/wiki/U.S._Highway_66_Association
"Behind the scenes, the federal aid program had begun with the passage of the Federal Aid Road Act of 1916, providing 50% monetary support from the federal government for improvement of major roads. The Federal Aid Highway Act of 1921 limited the routes to 7% of each state's roads, while 3 in every 7 roads had to be "interstate in character". Identification of these main roads was completed in 1923.[1]"
Also according to your source it looks like the U.S. numbered highways developed in spite of the numerous private organizations existing at the time:
"The new system was both praised and criticized by local newspapers, often depending on whether that city ended up on a major route. While the Lincoln Highway Association understood and supported the plan, partly because they were assured of getting the US 30 designation as much as possible, most other trail associations lamented their obsolescence. At their January 14–15, 1926 meeting, AASHO was flooded with complaints."
I feel like the reasonable middle ground here is simply what Fred Turner is insisting: the two components are intertwined and symbiotic. Why is that so hard to accept?