I have visited with the Custer County road crew.....their current belief/or received instruction is that if the road grader cuts deep enough into the road bed to remove the washboards (or corduroy); then, the road bed is ruined!
WASHBOARD: the series of evenly spaced bumps that make a vehicle chatter rather than bounce around like potholes
Cause: Road surface is tearing apart through the action of passing vehicles because the surface gravel lacks fines or adequate angular stone or moisture or a combination of the three.
Solution: Regrade the road surface to cut out the washboard and add good quality gravel with adequate sharp stone and fines. Treat with Calcium Chloride
Washboard is a direct function of traffic, especially fast moving traffic, usually on a dry road, regardless of whether it is upgrade or flat. Providing as hard and compact a surface as possible will help eliminate washboard. Good gravel with considerable fines helps to hold moisture. Calcium chloride increases the road's ability to retain moisture in dry seasons helping to consolidate it to help prevent washboard and other problems.
As I recall, didn't Custer County spend some over $80k for a rock crusher???