While it's incredibly hard to believe, it is true. If you look at population statistics for Philadelphia up to the 1980s, it fell to 4/5ths its peak in less than a couple decades.
Driving from river-to-river, or from city-to-suburbs or vice versa was a nightmare, according to everyone older than me. Driving "through" or "to" the center city's impassable traffic grid was hopeless. Then, in 1984, they took advantage of the government buying all the bankrupt railroads and used that property to create a below-ground highway Vine-Street Expressway for river-to-river traffic and connected Penn Railroads Center-city terminal with Reading's system through an underground commuter tunnel and turned Reading's terminal into the Convention Center instead of a giant mess off railroad track interlockings leading into Reading Terminal making traffic even worse. If not for that, I have no doubt it would be 3/5ths the peak population. I could write a 50-page paper on all the things it fixed and I was only 2 when it was all built.