"Harmonic rippling pattern" might seem like a description of the soothing output from a white-noise generator lulling you to sleep. That's close. But in this case, the noise machine is your car and the rippling pattern is the washboard road you're bouncing over. One listener wants to know what causes these washboard patterns.
Austin Amestoy: Welcome to the Big Why, a series from Montana Public Radio where we find out what we can discover together. I’m your host, Austin Amestoy. This is a show about listener-powered reporting. We’ll answer your questions — large or small — about anything under the Big Sky. By Montanans for Montana, this is The Big Why.
And joining us today for the first time in a while is MTPR reporter Victoria Traxler. Hi Victoria.
Victoria Traxler Hi Austin, thanks so much for having me. I can't wait to share what I've learned while researching this episode.
Austin Amestoy It always rocks to have you on the show, Victoria. What question do you have for us this week?
Victoria Traxler A listener in Helena, who wanted to stay anonymous, was curious about a problem I think we all experience while living in Montana, but is one that takes a specialist to fix. She told me about a time she was driving a gravel road near Paradise Valley.
Curious Montanan The road was very, very uneven, it was a real washboard road. And it really got me thinking, what causes a washboard configuration on the road?
Austin Amestoy Oh man, this could be the most universally relatable episode we've done yet, Victoria. I lived on a gravel road for 13 years, and it frequently wrinkled over with those washboard bumps. A couple of times I thought the car was gonna rattle apart.
Victoria Traxler Exactly. I can also say from personal experience that driving through Montana means getting familiar with washboarding.
Austin Amestoy So what did you learn, do we know what causes those annoying bumps on gravel roads?
Victoria Traxler You know, I was surprised to find that the answer isn't very straightforward. Physicists and engineers have been trying to figure out exactly what causes washboarding for decades. The best answer I found came from a 1963 study out of Alaska by a man named Keith Mather. Mather essentially determined that objects bouncing across a loose surface above a certain speed create irregularities in that surface. Each time a tire moves over those irregularities, it hops in the air and crashes down again. This develops and then accentuates the harmonic rippling pattern we know as washboarding.
Austin Amestoy That is the most flowery term I could have ever expected to hear ascribed to washboarding, Victoria. Does that mean those washboards are caused by drivers speeding over gravel roads though?
Victoria Traxler No, actually, and I was told this is one of the biggest misconceptions when it comes to washboarding. Yes, speed is a factor, but in order to prevent washboarding from happening, you'd have to drive less than 15 miles an hour, which just isn't realistic for people who live miles out on these backroads.
The key thing to understand about the cause of washboarding is that it mostly comes down to the stability of the gravel mix that the cars drive on. That's where people like Matt Ulberg come in.
Matt Ulberg I've always been a rural-roads guy. I like the two-lane, rural, paved or unpaved, gravel or dirt. The fewer the people and the more wildlife, the better.
Austin Amestoy He sounds like the right guy to ask.
Victoria Traxler Oh, yes. Ulberg is the director for Montana State University's local technical assistance program. It provides education and trainings on transportation. I'm not sure there's someone out there with more passion or expertise about gravel roads than Ulberg. He told me about this time his daughter was in kindergarten and they were driving on a main road that was overflowing with rainwater.
Matt Ulberg There was ponding on the road and my daughter was lecturing her friends on the poor roadway drainage and that the ditches need to be deeper and the culverts were not the appropriate size and that curb was an apron curb and it should have been a catch curb because all that water was spilling out onto the road, and she was a little expert at, you know, five years old and I realized that I probably talk about this way too much.
Victoria Traxler He went on to tell me that sometimes when he's driving, he'll actually get out and feel the gravel with his hands just to get a sense of the roadway. So to learn more about washboarding, I joined him on one of those trips.
Austin Amestoy That is the soundtrack of life in Montana. What did you learn?.
Victoria Traxler Ulberg says the recipe to mitigate washboarding is using three-quarter inch gravel – meaning rocks about the size of a nickel – and including a binding agent to that gravel mix. When you lived on that gravel road, did you ever notice a truck spraying something on the route?
Austin Amestoy I did. That's how they "fixed" our gravel road back in the day, and you know it did improve washboarding for quite some time, I remember.
Victoria Traxler Yes. They were spraying a common binder called magnesium chloride, which is a salt compound used to prevent excessive dust and washboarding on unpaved roads. However, too much of it can harm nearby vegetation and trees, so the next best binder is clay. Ulberg picked up a handful of gravel and dirt from the road we were on to show me how it works.
Matt Ulberg When you open your hand, you can feel the tension and the cracking. I can feel it as it opens up. That cracking as you open your hand is the tension of the clay. And that clay is sticky and it holds things together.
Austin Amestoy Fascinating. So if you use the right kind of sticky material, a gravel road won't begin to washboard?
Victoria Traxler Actually, no. It's an inevitable process, but the gravel-clay mixture helps prolong the lifespan of the road. That, in combination with regular maintenance from road graders, keeps a gravel road in good shape.
Austin Amestoy Adding clay and the right size gravel seems like a pretty simple solution. So why is it that we see washboard roads all over the state?
Victoria Traxler Well, a big part of the issue comes down to funding. Rural counties in particular really shoulder the burden of maintaining the largest percentage of the state's roughly 50,000 miles of unpaved roads.
Austin Amestoy Right the state's only in charge of the big highways and I 90 and I 15 and roads of that nature So a lot of that work goes to the counties then and rural counties don't have much of a tax base
Victoria Traxler Exactly. With fewer people, they have less money. I spoke with Powell County Commissioner Bill Hooker, who also worked for the State Department of Transportation. He told me that only about 50 of the county's 800 miles of roadway are paved, and all that gravel takes a lot of maintenance.
Bill Hooker The big challenge that we have, and we have every every single year, is finances and whether or not we can finance doing this.
Austin Amestoy So how do they manage washboard issues on so many miles of road? Wouldn't it be easier to just pave them?
Victoria Traxler Well, kind of counterintuitively, it's actually cheaper to maintain a gravel road in a rural area without a ton of traffic than it is to pave it. So with all that gravel road to maintain in Powell County, Hooker says he knows that washboard roads can take a toll on taxpayers' vehicles. They pose a serious safety risk, too. Like you said, the vibration and loose gravel can cause cars to go off-road pretty easily. But the amount of time and attention these roads get is really dictated by budgets.
Bill Hooker We had a $300,000 project in my district last year that we had to turn down because there simply was no money.
Austin Amestoy Wow, that does sound like a challenge. What are his thoughts on how to stay on top of it?
Victoria Traxler Well, he told me that even though the county is on a tight budget, knowing how to set up your roads correctly from the start sets them up for success in the long term. Through trainings like Ulberg's, Hooker began learning and implementing changes that might save money going forward.
Bill Hooker We're going to have washboards and we're going to need to deal with them. So then we need to learn how we're going to deal with these washboards.
Austin Amestoy So just to recap, washboards are formed from objects like a tire bouncing along the unstable gravel at a certain speed, and they are inevitable no matter what. But with the right mix of gravel and maintaining those roads correctly, we can help mitigate washboarding.
Victoria Traxler That is correct, Austin. And, you know, Montana has a lot of gravel roads, so maintaining all of them is really a challenge.
Austin Amestoy Indeed. Thank you, Victoria, so much for digging into this question.
Victoria Traxler Of course, Austin, it was my pleasure.
Austin Amestoy Now we want to know what makes you curious about Montana. Submit your questions below, or leave a message at 406-640-8933. Let's see what we can discover together! Find us wherever you listen to podcasts and help others find the show by sharing it and leaving a review.
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