I disagree about RTA comment. I take the 55 almost everyday and it is super clean. The city used to be dirty, but has been rejuvenated from public square to the east bank. It's really bike friendly as far as transportation goes as well.
Regarding the cleanliness, (parts of) the flats and public square have gotten better but that's only two small areas and is the extent of the cleanup. I drive past the steel yard every day for my commute and you can not have your windows down or let any outside air in all along 77 because the smell is so disgusting.
You might already know this but you're also driving past a wastewater treatment plant, and through a pretty sparsely populated area. It's not representative of the metropolitan area as a whole.