## Scientists have discovered why dirt roads develop ridges that make for a bumpy ride.

If you’ve ever been in a car that’s traveling down a dirt road, you know how bumpy the ride can be. Dirt roads often develop ridges—and until recently, no one knew why.

These bumps are usually several inches high, and they occur every foot or so. Workers can use bulldozers to flatten the dirt, but the ridges reappear soon after cars hit the road again.

Scientists have attempted to explain why ridges form, but their theories have been very complex. As a result, engineers haven’t been able to put the theories to the test or to design bumpfree dirt roads.

 As cars and trucks drive over dirt roads, they create ridges like the ones shown on this road in Australia. D. Mays

Recently, researchers at the University of Toronto and their colleagues at the University of Cambridge in England attempted to come up with a simple explanation of why the ridges form.

They began by building a turntable—a round, flat surface that rotates, somewhat like the spinning surfaces sometimes found on large restaurant tables.

To make a model dirt road, the scientists covered the turntable with grains of dirt and sand. They placed a rubber wheel over the surface so that it rolled over the dirt as the turntable rotated.

In repeated tests, the scientists varied conditions in every way that they could think of. They used grains of different sizes and mixtures. Sometimes they packed down the dirt. Other times, they scattered the grains loosely on the surface.

The researchers also tested wheels of different sizes and weights. They even used a type of wheel that didn’t spin. And they rotated the turntable at a variety of speeds.

Depending on conditions, the distance between ridges varied. But the ripplelike ridges almost always formed, regardless of what combination of factors the scientists used.

To better understand what was going on, the team created a computer simulation that showed how individual grains of sand move as a tire drives over them.

The computer program showed that dirt surfaces, even those that look flat, actually have tiny bumps. As a wheel rolls over these little bumps, it pushes the dirt forward a small amount. This nudge makes the bump get slightly bigger.

When the wheel then passes over the bump, it pushes dirt down into the next bump. After a hundred or so repetitions—not unusual for a well-used road—the bumps turn into a pattern of deep ridges.

What’s the solution? The only way to avoid a bumpy ride, the researchers found, was to slow way down. If all cars travel at a poky 5 miles per hour or less, a dirt road will remain flat.—Emily Sohn

Going Deeper:

Rehmeyer, Julie. 2007. Road bumps: Why dirt roads develop a washboard surface. Science News 172(Aug. 18):102. Available at http://www.sciencenews.org/articles/20070818/fob7.asp .