patchbunny, welcome to the Straight Dope Message Boards, glad to have you with us. Great first post!.. and we hope to see much more from you.
So, any challengers to the assertions of people who notice the same effect on stretches of paved roads, especially those in the right lane frequented by trucks? I, too, have noticed in certain areas, that effect.
No graders involved. What say you?
The washboards without question are caused by suspension oscillations. Any snowmobiler will be familiar with this problem. Well travelled snowmobile trails become terribly bumpy. A freshly groomed trail can have 2’ bumps after 100 snowmobiles pass over it. One bump turns into two, then three, until eventually the entire trail is one big washboard (miles long).