Washboarding is the name of the process which results in unmetalled (unpaved) roads developing a series of regular bumps with short spacing in the road surface. The result is called a washboard road, for its resemblance to the surface of an old-style washboard, or a corrugated road. It is sometimes informally called a "corduroy road", due to its resemblance to a road made of logs.
Washboarding is an instability that occurs when vehicles move above a critical speed that depends on the properties of the vehicles and the road surface. If all the vehicles move below their critical speed the road will remain flat, but if they move faster (or at least within a critical range), ripples will slowly grow and move in the direction of the vehicles. The vibration is felt most in the vehicle when the speed is such that some part, usually the wheels bouncing on the springyness of the tires, is in resonance with the bumps. It has been argued that the vehicle's suspension is important, but this can not explain why washboard road forms when vehicles' suspensions vary so much. Many have argued that suspension is irrelevant and recent experiments confirm that washboard roads form from vehicles without a suspension system. The onset of the instability is explained as the transition from dissipative frictional forces to conservative dynamic force  and the experiments also show that the pattern can move against the direction of motion or in the direction of motion. A similar instability also occurs on railroad tracks where it is known as roaring rails, and between rollers in machinery such as printing presses.
Washboarding cannot be prevented, but can be temporarily eliminated by regrading the road. The most effective way to permanently eliminate washboarding is through surfacing or installation of cellular confinement systems. Running lower tire pressures may also lead to less washboarding.
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- A.-F. Bitbol, N. Taberlet, S. Morris and J. McElwaine Scaling and dynamics of washboard road. Phys. Rev. E 79:061308 (2009)
- "Paving the road to the 21st century." American City and County, Nov 1, 1997.