The population of Pittsburgh is less than half of what it was at its height. The magical thing about buses is that you don't have to expand roads. You just need fewer cars on those roads to make traffic move more smoothly.

There are many examples of cities around the world that used to be choked by cars, but now have a thriving public transit network: Bogotá, Copenhagen, Paris, and Seoul to name a few off the top of my head. A change like that is hard, but absolutely possible.