What causes washboard roads?

Causes of Washboard Road Formation

Slowing down is one way to avoid the speedbump-like effect of washboard roads.
Tyler Stableford/Getty Images
Slowing down is one way to avoid the speedbump-like effect of washboard roads.

While washboard roads can occur anywhere, they are most often seen in hot, dry areas and on sandy, dirt or gravel roads. What causes washboarding to be worse in some locations than others? There are three main causes:

  • Lack of moisture -- Washboard roads are particularly prevalent when weather conditions are dry. Frequent rainfall reduces the chances of washboarding.
  • Traffic -- Hard acceleration or braking can accelerate washboarding, and that helps explain why you'll see it frequently at intersections, before and after sharp curves, business entrances, and even driveways. If a car's tires lose a firm grip on the road and begin to spin or skid, some gravel will be displaced. If this happens repeatedly, the gravel will be displaced, often uniformly, and a washboard pattern will form.
  • Poor quality surface -- Washboarding occurs most frequently if the surface quality of the road is poor, whether it's sand, gravel or even pavement. When a heavy load passes over the surface repeatedly, it develops irregularities like washboard patterns. Potholes are a common result of poor quality surfaces, too.

If you travel dirt roads frequently, you know that you can find an ideal speed that will help smooth out the ride. That's the speed at which your car's suspension system is pushing the car down at the same time you experience a dip in the road. The problem is that the more the tires press down, the worse the washboard depression becomes. It's a catch-22: your suspension system is giving you a smooth ride, but it's making the washboard problem worse.

For lots more information on cars, roads and auto physics, see the links on the next page.

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  • Hough, Harold "Dirt roads, washboards, and dust control." Miners News. February/March 2007. (Nov. 28, 2009)
  • "Science News: Scientists study 'washboard' road ripples." UPI. July 14, 2009. (Nov. 28, 2009)
  • ScienceDaily. "Physics Of Bumpy Roads: What Makes Roads Ripple Like A Washboard?" July 9, 2009.
  • Skorseth, Ken. "Preventing Washboarding." U.S. Roads.com. August/September 1998. (Nov. 28, 2009)