Another way it could end up in the pavement is if the gravel had it in to begin with.
Across the road from me there used to be a swimming pond with a concession stand that operated from the 20's to the 1950's. In the 1980's a gravel company started to mine it all off. By the time I got my detector in 1996 the beach area was all gone - just a bigger deeper pond now. Boy the coins that would have been in there. You see this place used to charge ten cents to swim for the day and they used to have buses bring people from the next city or two away over here to swim. I have seen pictures at my local historical society when the beach was in full use. I just wish I had gotten into detecting ten years earlier.
I doubt that the gravel machines would have kicked out the coins, so into the roads they went.
Happy Hunting,
Scott (MI)