That's only half the problem; you also need to reallocate housing from the diffuse suburban homes suitable for car-life to the concentrated cores of housing around the local stops in your mass transit system.
Which is not to say it shouldn't be done, but bootstrapping a mass transit system is a long, slow process that involves throwing away a lot more than our existing cars & highways.
> you also need to reallocate housing from the diffuse suburban homes suitable for car-life to the concentrated cores of housing around the local stops in your mass transit system
What if a significant fraction of the population doesn't want to live in such a concentrated environment?
Which is not to say it shouldn't be done, but bootstrapping a mass transit system is a long, slow process that involves throwing away a lot more than our existing cars & highways.