I finally stopped to look more closely. These ripples are about 10 inches wide ands look to be almost exactly uniform through long stretches of road. Where does this precision come from?
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My father operated a piece of heavy machinery for years that we called a “road grader” (I can’t be bothered to Google the official name). The machine is maybe 15-20 feet long and 10-12 feet wide. A long, narrow blade sits beneath the cabin area.
The blade pivots on an axis and also slides along a plane to extend across the surface of the gravel road. The blade also can be angled to scrape the surface (top edge leads the bottom edge) or push/dig the surface (bottom edge leads the top edge).
The washboard surface is due to an improper angle on the blade which causes the blade to stutter as it moves along the surface of the road. Speed is also a factor. Higher speeds increase the intensity of the ripple. If the machine moves too fast, then the rippling leads to whole sections of the compacted surface coming loose, which causes potholes.
If you’ve ever pushed a table across a floor, that vibrating sound it makes is the same principle of the blade rapidly moving up and down due to an improper angle. Pulling the table reduces the stutter (because the weight is slightly off the front legs) and slowly pulling reduces it even more.
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