What creates the “washboard effect” on dirt roads?

country roadA question from Viron Fessler of Gaston, Oregon. He writes:

“We live on a gravel road. Every summer, during the dry season, the road develops ‘waves’ or ‘ripples.’ The peaks of the waves are about 10 inches apart, with matching dips. These ripples can get around 3 inches high. What creates this effect?”

These are sometimes called “washboard roads” — and they’re the bumpy bane of many a country drive. It’s not completely understood how they form. But it’s thought that washboard roads form because of vibrations that go across the surface of your vehicle’s tire treads. These vibrations are a phenomenon of any moving tire. They originate with the vibration of a car in motion.

On perfectly smooth asphalt — and with identical tires — the vibration of the tires won’t affect the road. On asphalt, the treads make firm contact with the ground. But gravel roads will cause a tire to bounce slightly. And the instant the tread surface loses contact with the ground, it vibrates.

Loose particles of dirt are pushed one way or the other by this vibration. Similar tires moving at roughly the same speed reinforce this effect — creating a very bumpy “washboard” road. Once the washboard forms, there isn’t much you can safely do for a smoother ride over it — other than slowing down to a near crawl.

Forest roads in the tropics (Food and Agriculture Organization of the United Nations

What you have in your mind?