Costs can be reduced greatly even without any technological innovation. NYC and LA spend ~10x more per km of subway than what some European cities spend. (E.g.

Costs can be reduced greatly even without any technological innovation. NYC and LA spend ~10x more per km of subway than what some European cities spend. (E.g. NYC's $2.2B/km vs. Paris's $230M / km. [1]) So try to replicate what foreign cities are doing and you get significant savings. However since either way digging tunnels will be expensive, it seems inefficient to try to do this for cars. It would be more reasonable to just build a better subway system. Especially if in the future there will be automated fleets of self-driving cars waiting for you at each subway stop... [1] https://www.vox.com/policy-and-politics/2017/1/1/14112776/new-york-second-avenue-subway-phase-2 https://www.vox.com/policy-and-politics/2017/1/1/14112776/ne...