On the same day that SDSTAFF Hawk presented his explanation of the “washboard road” phenomenon, NPR presented a short “Earth Science” blurb about the same topic. Their explanation was a little weirder, saying that a tire exerting force downward (as a result of the spring expanding) forces dust particles forward and backward from the tire’s position.
I think Hawk’s explanation is much more reasonable, and jibes with my recollection of an article in Scientific American maybe 30 years ago. The main factor was the resonant frequency of the suspension system (or, more accurately, the average resonant frequency of the suspensions of all the vehicles travelling the road).
On hitting a stone, bump or rut, the suspension attached to the tire will flex either up or down, and begin oscillating. The shock absorbers will dampen this oscillation, as will any friction in the system, but not before it delivers its varying pressure signal to the road surface.