People (including myself) are already saying this.

I live in an Easter European major city and when traveling to a civilized country (yes fellow eastern europeans, hate me for saying this) I'm always take aback by the quality of the air. I've recently visited Stockholm and the amount and variety of mushrooms growing in small urban parks surprised me -- I'm sure there's many orders of magnitude less pollution there than where I live.

I always think about this even today when an older car passes by - you can really smell the difference between 1980s exhaust filtering and more modern technology, and imagine how it must have been when all cars smelled that way. So technology is improving, but I'm not optimistic that we will get completely rid of motor traffic in cities in my lifetime...