Like James Bond's martini

I used to think that the big machines out grading dirt roads were intentionally making the washboard pattern for increased traction. 

Why else would there be perfectly even waves in the road?

In fact, the machines scrape and loosen the dirt breaking up the wave pattern which gives us a nice smooth ride for a day or two.

Here's an explanation

Here's another

Tim's approach is confirmed by Myth Busters (Episode 58)-

 driving faster is smoother.

However, as I drive slower to accommodate my old tires, 

I've wondered if it could shake cream into butter. 

Nothing so helpful has occurred to date.