The tendency of unpaved road surfaces to develop lateral ripples
(``washboard'' or ``corrugated'' road) is annoyingly familiar to
drivers on dry gravel roads. Similar ripples are well known on railroad
tracks and many other rolling or sliding, load bearing surfaces. Our
approach combined laboratory experiments and soft-particle direct
numerical simulations. The experiment consisted of a rotating table 60
cm in radius with a thick layer of sand forming a roadbed around the
circumference. A 6 cm radius hard rubber wheel, with a support
stationary in the lab frame, rolled on the sand layer. We varied the
speed of the table and the details of the suspension of the wheel. The
onset of the ripple pattern exhibits a sharp threshold as the speed is
varied. The ripple pattern appears as small patches of travelling waves
which eventually spread to the entire circumference. The ripples move
slowly in the driving direction. Interesting secondary dynamics of the
saturated ripples were observed. All of these effects are captured
qualitatively by a 2D soft particle simulation. The simulations clearly
indicate that neither compaction nor particle size segregation are
crucial for appearance of the ripples.