> I suppose one could argue that cities in the early - mid 20th century weren't as heavily populated. I could be wrong, but I would also assume that noise levels were much lower and the streets much less congested.

That is wrong I think. Inner London for instance still hasn't recovered the population level that it had in the 1930's. There are also videos that show a lot of traffic, and huge numbers of pedestrians. Go back another 30-50 years and most of the traffic would have been horse-drawn carts, which means enormous amounts of excrement, and extreme levels of noise, because before the invention of pneumatic tires wheels were metal on stone.