Why Roads Get Washboards

For as long as we’ve had clothes, we’ve done laundry. Long before detergent pods and spin cycles, people figured out the simple trick of  rubbing textiles against rough surfaces, which squeezes dirty water out of the fibers and pulls  cleaner water back in. Roughly around the 1800s,

that idea got a dedicated tool that you still  occasionally see today, even if only in jug bands. The washboard spread across the world as a faster,  more practical way to scrub shirts and pants. Its use continued to grow until automated electric  machines took over in the early 20th century.

But we’re not here to talk about laundry. That  era wasn’t just the heyday of the washboard. It also marked the start of the highway era (at  least in the US), when road construction boomed, and traffic skyrocketed, including on the unpaved  routes that connected farms, towns, and job sites.

If you’ve ever driven on a dirt road, you know  one of their biggest problems. In some places, the surface organizes itself into a repeating  pattern of ridges and valleys that can rattle your teeth, shake your suspension to pieces,  and make the vehicle feel like it’s skating.

It’s obnoxious at low speeds. At higher speeds,  it can become legitimately dangerous because bouncing tires can cause you to lose control of  steering and braking. In 1924, this phenomenon was known as “rhythmic corrugations.” By 1929,  people were calling them “washboard” roads.

And nearly a century later, they’re still  a persistent problem all over the world. What’s surprising is that this is still an active  area of research. In the past decade or so, there’s been a burst of papers trying to explain  exactly how washboarding forms and how to stop it.

I was reading one of those studies when  I saw the experimental setup they used. The moment I saw it, I knew I had to build one.  Let’s talk about why this happens, and why there’s still more to learn about washboarding. I’m  Grady, and this is Practical Engineering.

Roughly 35% of the countless thousands of miles  of roadways in the United States are unpaved. That’s technically true, but a little  misleading since most of those roads are in rural areas that carry only a tiny  fraction of the traffic volume compared

to the roads paved in asphalt or concrete.  This is not a measured or reported statistic, but some back-of-the-envelope math shows  that only about one percent of traffic happens on unpaved roadways in the US. That’s  not true in other parts of the world, though.

We call that outer layer of a road the wearing  surface (for obvious reasons). Of course, using a hard material like asphalt or concrete  as the wearing surface has a lot of benefits: it cuts down on dust, it’s generally  smoother and more comfortable to drive on, it’s easier and safer to drive at high speeds,

and it’s more durable. But as good as it is,  paving doesn’t make sense in every situation. Almost everything in transportation engineering is  an economic decision. Roads seem kind of mundane and simple, but you have to consider the scale.  Think about a basic home improvement project like

installing new flooring in a kitchen. Maybe it’ll  cost you a few thousand dollars. Then consider that a mile (or 1.6 kilometers) of two-lane  roadways has, very roughly, 600 kitchen’s worth of surface area. For a simple commute to  work, we’re talking about thousands or even tens