We call these roads here metal or shingle roads.
They sometimes do get called wash board roads too because of the bumpy corners.
Graders roughen up the surface once in a while.
I think the channels are caused by run off from rain water, following the camber of the road if any, otherwise potholes appear.
I think washboards are also sometimes caused by slipping differentials
Interesting yaktx, I have never thought of that but I think you are quite right there, thanks, I learned something new today about the "SWER" roads.