That I can remember right now, I've taken the subway/train in New York, Boston, Chicago, London, San Francisco, and DC. Some of them are really great, others are not (looking at you, Green Line on the Boston T). Building these systems was almost certainly a great idea at the time.
My main point is that technology is now changing and I think that de-centralized mass-transit through fleets of automated cars is close at hand. And it can use the infrastructure that we already have. So building super-expensive fixed-line transit infrastructure within cities is not a good investment anymore. Even more so when you consider the turnaround time from initial planning to opening to the public is probably a decade.
Private transport takes up way too much space and is also inefficient. Why spend the energy and resources to build a 1-2 ton car for transporting 1-2 people? It seems like an odd idea.
1. You wouldn't need a car per person, only enough to comfortably serve peak demand. There's never a single point of time at which everyone in a city is on the road. (Except evacuations, but see point 3).
2. There is already about one car per person in the US.
3. Automated car fleets would presumably have a cheaper carpooling option, where the car picks up other passengers on the way to the destination. Like UberPool today.
4. Fixed-line transit costs millions to billions of dollars per mile, and easily takes a decade from planning to completion. Building roads takes a fraction of the time and money. And the roads already exist.
Automated car fleets work well for suburbs. But past a certain point, heavy rail train subways have far more throughput than the equivalent amount of cars.
Building & maintaining roads and highways does take a lot of money too. Building new roads vs building new railroad track on new land isn't that much different cost wise.
My main point is that technology is now changing and I think that de-centralized mass-transit through fleets of automated cars is close at hand. And it can use the infrastructure that we already have. So building super-expensive fixed-line transit infrastructure within cities is not a good investment anymore. Even more so when you consider the turnaround time from initial planning to opening to the public is probably a decade.