I'm not sure if you mean that Chicago is clean or dirty? I saw a lot of both last time I visited, it's hard to say. It certainly seemed a lot cleaner than, say, Rochester NY or Philadelphia when I visited, but not sure if that was just the areas I was in.

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.