I heard a civil engineer make a claim once that the dust on the side of the road is about 300% more laden with precious metals like platinum, than random mining. I suppose this is all roads and not just American roads, though.
Cody's Lab did a video with some experiments collecting and refining road dust. As I recall, he did manage to obtain a small bead of platinum-group metals but it didn't appear to be economically viable at least at a small scale.