This may be a silly inquiry, but I have often wondered why parts of many concrete road surfaces contain a groove which usually waves back and forth along my direction of travel, from one side of the lane to the other. The groove may last for a mile or two, or sometimes several miles, then it may stop. I don't notice them while driving my car, but they are aggravating on a bike, and when a groove seems particularly deep I find myself riding from one side of the lane to the other, trying to avoid running over, or along, the groove. I couldn't find a thread on the subject on this site nor could I find an answer from a Google search. Are there any road construction/engineer types who could tell me why these "semi ruts" exist?