>Self-driving, solar cars solve all problems of mass transit and are far cheaper, since they use existing roads.

They don't solve the arguably most important issue: road capacity. A bus that takes the same space on the road as 2-3 cars seats 30 people (and holds another 30 standing). Self-driving cars enable somewhat more efficient traffic patterns, but they will have a hard time overcoming the order-of-magnitude lead that buses have.

And that's not even mentioning subways, those don't impact roads at all and move staggering amounts of people