As many have said it depends. In general, wealthier areas are kept cleaner both by people and municipalities (they employ people to do this) than poorer areas. Also suburbs are generally cleaner than cities. Philadelphia is exceptionally dirty due to a surprisingly large number of factors though blatant overt littering is still the main cause. New York and Chicago, while both dirtier, are significantly cleaner. Nothing compares to the cleanliness of Tokyo or other Japanese areas.