I lived in New Haven for 4 years. The difference in the amount of trash between American cities is vast, compare NYC with Chicago. Or really any city on the east coast with Chicago for that matter.
Of course the difference is vast, I think you could compare one part of NYC with NYC even and end up with vastly different ideas of trash. And that's kinda what I am going on about; the trash under highways in some part of the city is a meaningless metric.
I was flying from NYC to Denver a few months ago and a Denverite and I struck up a conversation on this very topic after observing multiple people throwing trash on the floor in LGA. Speaking of which, Denver, another city that is much cleaner than anywhere on the east coast.