Why Roads Get Washboards — Practical Engineering

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[Note that this article is a transcript of the video embedded above.]<br>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.<br>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.<br>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.<br>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.<br>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.<br>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 of thousands of kitchens now. This is silly, but the point is that roads are extremely expensive, not because they’re particularly sophisticated, but because they’re huge. You don’t get a monthly “road bill,” but if you add up gas taxes, tolls, and the slice of general taxes that fund roads, the average household in the US is paying roughly on the order of an electric bill each month. So engineering decisions about roadways are largely driven by costs. Here’s a simple example:<br>An unpaved road is pretty cheap to build. Before we consider the traffic, it sits pretty low on this graph. But as traffic increases, so do the maintenance costs associated with regrading, adding new gravel that’s lost over time, controlling dust through chemical application, etc. Its fixed cost is low, but its variable cost is steep because it’s not as durable. A paved road is expensive to build initially, but it requires less maintenance and that maintenance is less sensitive to traffic volume, so its variable cost is relatively flat. You can see there’s a breakeven point based on traffic volume, below which it makes more sense not to pave a road. And that’s just looking at traffic. When you factor in other complexities like the availability of materials, the abundance of skilled contractors who can do the work, and climate factors, there are plenty of situations where paving a road isn’t the best economic decision. In many countries, pavement is a...