system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do dirt roads develop a washboard texture over time?
gpt
Start with a smooth surface covered with something granular like mixed dirt and gravel. 

The first car goes over it. Its wheel bounces when it hits a pebble or a spot that isn't quite as hard-packed and even as the rest. That bounce creates a little divot and then a bump, and the car bounces along a few times. Some gravel is kicked back and forth on each bounce.

The second car goes over that area, not quite on the same path but close. That car misses the little pebble but catches a part of the first bump and divot, and that part causes the car to bounce too. More bumps and divots are formed by the bounces.

The third car goes over. It hits the bumps and divots squarely, making them larger and then adds a bunch more on top of those and stretching out that area of creases.

As more and more cars go along that road, the already-existing bumps get amplified into ridges that cross the whole road. Some of them get wiped out by people driving at different speeds or braking and stuff, sure. But eventually the pattern becomes pretty entrenched. And if it's on clayey soil it can get worse if you add rain and heat, which might bake the ridges into place and wash away looser material in the divots. 

Eventually you end up with washboarding, which can stick around until a grader comes and levels out the road.