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

I have suspected that they come from ABS brakes since I don’t remember seeing them when I was young. And they show up in snow near red light