That's averaged over weeks or months or however long it takes for repairs, and over however many thousands of cars pass by (the US drives trillions of miles each year).
Detail:
I'm speaking in generalizations here, but it seems that small and even less traveled roads that have pylons often have these damages.
Scarily, on a run today, a "lightly" traveled road in a residential area had multiple "stop for pedestrian" signs that had been knocked down. I find this particularly scary on the (non major side street) that I live - where other than residents (300ppl) and 1 business (< 300 ppl) few people should choose to drive - loses these road pylons in a matter of days.