True, but the radius affected by airliners is generally much smaller. Once they're higher than a few thousand meters, you can't really hear them any more.

Cargo trains, I imagine it's similar.

> Congestion is too severe for anyone to make significant noise, unless they have modified/dysfunctional exhausts or particularly large engines.

Large numbers of cars idling make enough noise to basically rival human conversations at regular speech levels. Anything above that (usually anything higher than 30kmph) makes it even worse. I'm not sure it's exponential, but I think it increases supra-linearly afterwards.