I said "WHEN our driverless cars head underground". I did not say all the travel would be underground. Small single passenger vehicles can also pass each a lo

I said "WHEN our driverless cars head underground". I did not say all the travel would be underground. Small single passenger vehicles can also pass each a lot more easily in the amount of space a train takes up. You could have two levels in the amount of vertical space a train takes up. I also don't think a train takes up less of the available space and time than individual cars would, when has to stop at all the stops. All other trains must wait too. The flow is constantly interrupted. Robotic underground construction is also going to be a lot cheaper. Whatever happens and whatever the mix between walking and vehicle transportation evolves, people are going to prefer to travel in their own vehicles over being crammed into cars with strangers and having to navigate escalators. This particular system is shaped by the current technological constraints, not so much by people's desires. When technology advances, it will eventually change.