I think the idea is you have a subway that runs the length. One set of trains in both directions that are express and another set with lots of stops when you only want to go a couple of miles up the road, say to work. You wouldn't have to tunnel, just dig a ditch. Then have a park in place of the street, perhaps with bike lanes. Any cars would be parked just outside the city but still within walking distance. I personally think it's an interesting idea and hope it gets built.
Detroit is rebuilding the city around neighborhoods. The idea is you reinforce neighborhoods around the idea that most of what you need is no longer than a ten minute walk. They've built greenways connecting places to make this possible. Kind of ironic for the city of cars?