There is plenty of capital in the world right now that there is no reason not to build all different kinds of new transportation schemes. Elon's first tunnel got NIMBYed. One main feature of tunnels is that you can, hopefully, put them lots of places where surface building is impossible. What people put in them will be up to the builders.

Construction costs has not come down in price over time like many other things have. I think he hopes that the Boring Company can list a price/mile for tunneling that is much lower than current costs and let people actually budget them. Maybe sell these better machines to other construction companies at some point. He broke the cost-plus pricing for satellite launches. Just think what something like that for tunneling could do for transportation systems. Billion dollar overruns are on the digger not the public. That would be great.

If you actually look into construction projects, including extremely expensive American ones, tunnel boring is not a significant cost.

The actual cost comes in station digging; stations need to be high throughput and extremely reliable, which favors complexes with a large footprint and thus more required property taking, usually in dense urban areas that require complicated engineering. For highway tunnels the similar cost factor are onramps and offramps; if you want a ramp to be safe and efficient at high speed you need a lot of space.

Hyperloop “solves” this problem by proposing elevators. Very few metro stations are elevator only because they are low throughput and are prone to mechanical failure, as opposed to staircases that are not.

An interesting writeup on the case study of the Second Avenue Subway: https://pedestrianobservations.com/2018/01/27/construction-c...

Yes. With cheap tunnels one could hope to plan for many exits and a large network that goes everywhere. Like a road system, but underground. Imagine an early city where all the roads are really just paths in the dirt that are completely rutted and almost impassible when it rains. Someone proposes to pave the main road with cobblestones. This is really expensive and wont help people travel on all of the other paths that still won't be paved.

Ok. Give up I guess; bad idea.

No. Let's start with this one, fix the various problems that arise, discover how to make a good paved road with the least amount of labor and resources, and continue to expand the paved network to reach everywhere in the city.

This is what happened. Would you want to go back to unpaved roads in a city? Traffic congestion is killing American society. I like American society. Many people don't like to live all packed together but still want to work with other people and have the amenities that cities enable. Cars and interstate roads allowed that for a few decades but now we need a new system to continue expansion of the free flow and interaction of people. Underground roads could replace interstates and we could turn them into grand boulevards for pedestrians, bikes, and other slow vehicles. Wouldn't that be great? Almost all car traffic is underground. Would you support such an idea?