Washboard also refers to road corrugations.
Road corrugations are formed when the wheel of a vehicle hits an imperfection on the surface of the road. This imperfection, whether it be a rock, a clump of mud or roadkill causes the wheel to bounce upwards, removing pressure from the surface of the road. The wheel then bounces back down to the surface with a velocity of such that it creates a depression on the surface of the road. This process repeats with every wheel that crosses it and thus creates the washboard effect.
This is most noticeable on dirt, clay, cinder or gravel road, but also occurs on paved roads and even can occur at a less noticeable level on steel.