There were about four Golden Ages of highway driving. One was that transitional period, when the car was first getting market penetration, and the notion of driving coast-to-coast was still adventurous, but not suicidal. That's this era. Rich and Powerful Men could dream of uniting the country through a highway, the way their grandfathers had driven the Silver Spike. They weren't thinking about comfort, yet. They were just trying to get the road built.
Highways were mostly regional affairs, and these markers were how you knew you hadn't wandered off down some rancher's driveway. There were booster associations, advertising the routes to get the tourist business. That's how the "Lee Highway," the "Jefferson Davis Highway," and the "Lincoln Highway" got started. But they were regional affairs all over the country, so the First Great Paving really didn't take all that long. I have a book from the 1930s, Double-Crossing American By Road, about a round-trip car trip taken by some New Yorkers. Half the talk is about the price of hotels and where the good restaurants were. By then, making it was almost a given.