http://www.straightdope.com/mailbag/mwashboard.html
I think I have noticed a similar effect on paved roads.
Has anyone else seen a regular pattern of potholes, generally of decreasing size, particularly on interstates (where the range of speeds is reasonably narrow)?
On mountain roads, and especially dirt roads, they stagger-grade the road with the grader in order to prevent slipping down inclines.
Minor point: The weight of the car only indirectly affects the free bounce frequency of the wheels. The wheels naturally bounce up and down at a rate that is determined by the wheel mass and the stiffness/damping of the suspension. The car basically sits still compared to the wheels whether you’re talking about a Buick or a Yugo. However, a heavier car will tend to require a stiffer and more heavily damped suspension, as well as larger and heavier wheels, so this indirectly affects the natural frequency of the wheel bounces.
You guys have overlooked another possibility- “washboard” ribs along a road can also be created by a tracked vehicle (bull dozer, trackhoe, etc.). Think about what the tread of a tracked vehicle looks like, then try to imagine what kind of pattern such a tread would leave behind. The treads themselves can create the pattern in soft or moist soils, but the tread also tends to compact soil in that pattern, so that even though the road looks normal after the vehicle has passed, once the wind/rain/snow/etc. has acted on the soil for a while, the less compacted material is washed away, allowing the pattern to emerge long after the vehicle has come and gone.
Having driven on many miles of the above mentioned roads and knowing a grader operator personally, the “washboarding” is almost always caused by applying the blade to the road surface nearly perpendicularly and not at a steeper angle. This causes the blade to “skip” as it is pushed over the surface instead of “skinning” the surface at an angle thereby pushing the newly scraped surface to the side.
Try this: using a ruler, place it on a piece of paper, lean it back about 45 degrees and make sure it is square with the paper (not at an angle). Now push the ruler across the paper at a right angle to it’s position on the paper. It drags and skips.
Now using the same 45 degree angle, reposition the ruler so it is at about 30 to 45 degrees to the direction of travel. It will “scrape” the paper without “skipping” or dragging.
I have noticed washboard paved roads around here. On the asphalt roads, the right two lanes, where the trucks are restricted to go. Gets worse over time. Definitely nothing to do with graders skipping.
Also note that the washboards propagate across the road, getting wider. Two narrow strips of washboards first appear. People find it bumpy driving over those strips, shift over a little. But they still sometime overlap the old ones, the bumping on the overlap helps extend the bumps sideways. Since there is not center line on gravel roads, and little traffic, people drive all over the place. The bumps can extend uniformly from shoulder to shoulder.
Regrading/filling washboard gravel roads usually is only very superficial so the originals tend to reappear quickly.
Another possibility that I’ve seen in both rural Wisconsin and in the Pacific Northwest is that the roadbed was once “courderoy,” laid back in logging days from tree trunks.
I used to do a lot of dirt-bike riding on desert roads. These were not roads that were used a whole lot. Since they were in the desert, and the desert is a windy place, would it be that “washboard” roads are simply caused by the wind in the same way rippled surfaces are caused in stream beds?
Well, I’ve driven on these washboard dirt-roads more often than I like and the most common explanation I’ve heard is that they form because the wheels are spinning a littles faster (or slower) than the car moves. This causes the road to move into a small hill which causes the next car to lift a little and it’s wheels to spin and cause a new hole when it lands and move more dirt. This is why washboard form behind potholes and in curves and on uphill streches of the road. Also on both sides of bridges and gates.
The only thing to do, if conditions permit, is to speed up until you “float” over the potholes/washboard, but that can make steering a bit difficult, but OK if there are not sharp turns and no other traffic. Also because you’ll going fast, sandpits in the road will be really dangerous.
I don’t know if it’s the same phenomenon, but I noticed regular waves perpendicular to the road’s direction in very old roads in the Adirondacks. My grandfather tells me that these are the outlines of the logs which were laid down as nice, even, stable bottoms for the road. The spaces were filled with sand or something, then the whole thing was covered in multiple layers of dirt and rock and gravel. Now the sand is slowly being eroded away wit hevery rain, every so slightly faster where the sand is deeper (where the wood isn’t). Take a millimeter every storm and soon you’ve got several miles of very interesting driving.
P.S. I’m glad to say that many of the trees in the Adirondacks have again reached the size of those logs in the road. Viva la forest!
Some roads make their own washboards. They do sit-ups and crunches when nobody is around.
<< Since they were in the desert, and the desert is a windy place, would it be that “washboard” roads are simply caused by the wind in the same way rippled surfaces are caused in stream beds? >>
Such an explanation is possible for the roads you’re talking about, but then the pattern of ripples would be irregular. If you’re talking about a regular, repeating pattern, then it can’t be caused by a random phenomenon like wind, water, or bouncing graders.
Certainly, if there were logs underneath a road, that could explain the erosion pattern for that road… but I know damn well there were no logs under the Iowa dirt roads.
You can learn a lot on an Iowa chain gang 
I am most familiar with washboarding on dirt/gravel roads in arid areas (certainly no trees around to put underneath). Since the washboarding is always perpindicular to the road direction and never changes wrt prevailing wind direction, you can pretty much rule out wind as a factor. Washboarding parallel to the direction of the road after turning 90 degrees would be a very odd sight.
I have to respectfully disagree with the always-well-informed C K Dexter Haven on this one:
Seemingly-random phenomena certainly DO produce repetitive patterns in dynamic systems, man-made or natural. As Johnny L. A. noted, everyone knows that wind causes wave-like ripples in sand. Just a small-scale result of a temporarily-stable wind velocity on an almost closed system? Well, the Sahara desert has been studied in regard to this phenomenon, (looking for link to this; sorry for now) and investigators have noted that it happens on multiple scales simultaneously, from mere inches across between cycles, to hundreds of yards. In other words, at your feet is one small pattern of ripples, extending up the face of another, similar pattern MANY levels of scale larger–while these in turn are found in and amongst giant repeating dunes the size of houses, but which have nevertheless been formed with great wave-like regularity and with self-similar morphology. Wind direction and velocity might vary a bit–sometimes quite a bit, but the patterns still emerge, seemingly because of the rough similarity of the size and mass of the sand grains being displaced.
Apparently, the wind picks up each sand grain and carries it downwind a distance which. understandably, varies directly with the sand grain’s mass. Sand grains are fairly regular, so this distance is almost the same every time, let’s call it n-distance, for every grain moved. Very interestingly, the wind speed seems not to affect this relationship, if I remember correctly. A regular harmonic system so far, yes, but even so, why doesn’t a flat desert stay flat?
Understandably, all it takes is ONE disturbance to kick off a rippling pattern’s development, like a stick, or an animal carcass, or even a rogue sand grain too heavy for the wind to pick up. Sand builds up against or is deformed by the pertubation, and the grains which are highest are picked up by passing wind, and deposited n-distance away. More grains are blown into position against the original disturbance, taking their place and ultimately being lofted, again, n-distance away, essentially right on top of the grains displaced earlier. Meanwhile, all these new grains sitting downwind from the disturbance are now sticking up higher than the area where they were deposited. The wind picks them up again, and, you guessed it, deposits them another n-distance away, or 2n from the origin point. And so on, and so forth, forces conspiring to build up a repetitive, harmonic pattern which can flow across hundreds of miles, although we in temperate climes are used to seeing them in only short bursts, due to the disruption of the system and the intervention of streams, embankments, blah blah blah.
I apologize if everyone already knows the above, but I remember being fairly fascinated by it when I ran across the info some time back. In any case, it is for these reasons that I have some curiosity (and doubt) as to whether the culprit really is the harmonics of a car’s SUSPENSION packing down gravel in the formation of washboard roads. It could just as easily be that gravel is being kicked up by the passing cars and distributed “downwind” n-distance, with great harmonic regularity, just like sand grains, but with mass scaled up. and the resultant n-distance scaled down as a result. Gravel is very regularly graded, after all, so it doesn’t seem out of the question to submit it as an analogue to the Saharan sand system.
Anyway, a washboard could conceivably be generated by ONE very-hard-to-discern irregularity or “strange attractor” in the system. After all, it seems pretty hard to predict just where the rippling seems to promulgate from, apparently just popping up in the midst of fairly uniform stretches of gravel. But perhaps one tiny dip or groove kicks the whole gravel-distributing cascade into motion, and if so, that would happen whether cars had particularly harmonically-resonant suspension systems or not. Just my two cents.
Cheers to all you good people,
-Shawn
This seems an unreasonable comparison to me. Gravel spread across a 12 ft. wide road, being acted upon by the tires of passing cars isn’t as uniform as the wind blowing grains of sand.
Well, of course you might be right, but how do you know? Evidence or theory ruling it out? I’m not saying you’re wrong, but I’m just not sure why you’re saying I am.
As I noted in my post, most folks would think, as did the fine C K Dexter Haven, that interaction with so-called “random” systems like the wind, water, etc. are too dynamic to produce regular feedback loops. This seems to be what you’re getting at too, but I don’t know how you’ve come to that conclusion. Forces may be more regular than you apparently presume…
Well, as one who designs highways for a living (though admittedly, not gravel roads), I’ll contribute my thoughts on the matter. My reference is the “Gravel Roads: Maintenance and Design Manual”, South Dakota Local Transportation Assistance Program, 11/00.
Washboarding has three primary causes: driving habits, lack of moisture in the roadbed, and poor gravel quality.
Driving Habits: Hard acceleration/braking are a major cause of washboarding. This is why you’ll see it along curves, hills, and intersections. I suspect that it’s a combination of reasons discussed before: the bouncing of the car and the shear forces exerted on the roadway by the tires. The problem is amplified by:
Moisture: If the roadway is dry, the roadway surface can lose cohesion and break apart. The stone and sand-sized particles can then “float” and shift into a washboard pattern.
Gravel quality: If you don’t have a good gradation of particles, and your fines have poor plasticity (which helps the whole mess bind together), the roadway will have trouble staying intact.
Dixson2 is correct in that poor construction techniques can also cause washboarding right form the outset. If the grader is moving too fast, or if the grader has improper tire inflation, it can generate a washboard surface during construction.
Then why, around here, does washboarding show up only after a heavy rain?
I suspect the reason is the constant bane of traffic engineers: water removing fines from the structural section. The rainwater carries off the small particles from the roadway, creating voids which allows the larger stones to shift about under traffic loads. Too much water is bad for roadways, too. 
I have a culvert on one project that is experiencing “piping”, i.e., water is flowing along the outside of the pipe. As the water travels, it carries along the smallest particles. This makes holes that allows water to carry along larger particles, and so on, and so on. The roadway, as a result, is collapsing into the rather large void being created.