No, that’s not a wave, because it doesn’t travel under its own energy. So long as the buses keep coming in the same way, the wave just gets bigger.
Washboarding is a result of bouncing by vehicles with powered wheels; so long as a car’s drive wheels can bounce upwards from collision with a bump, it will make washboards. The basic mechanics works like this:
The car is traveling along the road. A drive wheel hits a bump; the wheel bounces upward in response. What goes up must come down, so the wheel comes down a smidge later, but by the time it comes down, it has moved forward a short distance. There it hits the road with more-than-usual force, digging in a little harder and, because it’s a drive wheel, pushing a small amount of dirt backwards, right onto the bump that caused it to bounce earlier. That makes the bump higher and the hole following it a little deeper. But now it’s deeper than the normal road surface, so the road surface in front of it looks like a bump to the wheel, so the wheel bounces up again. And so the process repeats.
All of this is affected by a number of variables: the speed of the car, the strength of its shock absorbers and springs, and its weight. And in fact, hitting a washboard at different speeds results in different behavior—but most of it unpleasant. The one speed affect we can be certain of is this: if you drive slowly enough, the drive wheel doesn’t bounce but instead just roles smoothly up and down the bumps. But that usually takes very slow speeds. And if you’re going uphill, you need to maintain a minimum speed or the engine will stall—which is why washboard roads are worst on slopes.