Some experts who maintain dirt roads say that your car's suspension system causes the problem as it actually tries to smooth out the bumps in the road [source: U.S. Forest Service]. As a wheel moves over a bump, the suspension system absorbs the shock and then pushes back against the road surface. On a soft surface like a dirt road, the push back either packs or displaces the dirt it hits. Over time, as more and more cars go over the bump, the washboard pattern develops.
However, in the summer of 2009, physicists from Canada, France and the United Kingdom published a new study about the physics of washboard road formation. They discovered that ripples will form, even when the springy suspension of a car and the rolling shape of a wheel are eliminated [source: American Physical Society].
They built an experimental vehicle, replacing the wheel with a suspension rolling over a road with a simple inclined plow blade, without any spring or suspension, dragging over a bed of dry sand. Ripples appear when the plow moves above a certain speed.
After observing the results, they compared this phenomenon to the physics of stone skipping: A stone needs to be thrown above a specific speed in order to have enough force to bounce of the surface. A washboarding plow is similar, except the sandy surface remembers its shape and the effect is amplified.
They concluded that the formation of a washboard pattern is inevitable. The ridges will form, even if the wheel diameter, suspension or surface is changed. The only way to avoid the effects of a washboard road is to stay below a certain speed, but that's impractical: you'd usually have to drive at 3 miles per hour (4.83 kilometers per hour) to eliminate the problem altogether.
Some day, such discoveries may lead to improved suspension systems or improved road surfaces that smooth out a bumpy ride. In the meantime, there's not much you can do about washboard road except brace yourself, watch your speed and hang on for the ride.