I think what we may end up seeing replacing "mass transit" as we know it... will be universalizing self driving technology so vehicles can operate in closely packed "road trains". Your car would be able to join road trains and leave them but would offer safety by coordinating traffic in more manageable blocks.
That still doesn't address the fact that each of those self-driving cars will have its own engine that requires fuel and/or its own battery that requires power from somewhere. In that regard, they'll never be as energy efficient as mass transit.
