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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There are several mechanisms at play, all simultaneously.
First is the angle of stacking for the local sand/dirt. For most sands/dirts the angle is just in a sweet range to allow dunes to form (which is all washboard is, really small dunes), and so the very wind itself will help enhance existing washboard. Flowing water with particulates will do this too, it’s a function of ripples, vortexes, and hydro/aerodynamics. This includes the moving air caused by fast moving vehicles going down the road, so to slow the formation of washboard GO SLOW. Won’t stop it, but it will slow the formation.
A second mechanism was mentioned elsewhere, the “bow wave” effect. Underinflated tires will push loose material forward until it reaches a particular density, at which point the tire compresses slightly and rolls over the material. No matter how inflated the tire is, this will still occur over time, even completely solid “tires” will do this, they simply just move up and over the newly formed ridge instead of compressing slightly.
The third mechanism is a compression/bounce effect. A tiny, unnoticeable to you, bump forms in the surface, could be as simple as a pebble in the road, and forces your wheel up ever so slightly. This compresses the ridge, making it more dense and more likely to resist the NEXT tire that comes across it, but then the tire slams back down on the far side of the ridge, pushing material out of the way both forward and back, creating a tiny new ridge and reinforcing the first one. Repeat this process hundreds of times and the ridges cease being unnoticeable, you really end up feeling the washboard.
There are other mechanisms happening too, but those are the biggest ones, it’s all about the same things that create dunes and ripples, just on a smaller scale.
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