What causes the even pattern of ripples in a “washboard” gravel road?

Question

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?

In: 879

Answer 1

“According to the Federal Highway Administration (FHWA), the four primary causes of washboarding are driving habits, lack of moisture, poor gravel quality, and lack of crown on the road surface.”