[maxharris]

On a heavily-used block with tall buildings on both sides, here's a recipe that will work well:

1. Dig up the street and sidewalks, going down 60-100 feet.

2. At the bottom of this pit, drive in piles if you haven't yet hit bedrock.

3. Build a box, about twenty feet high, to run plumbing, power internet, and other utilities

4. On top of that box, build another one for a train.

5. On top of that box, build another one for vehicles to pass through. Because we're aiming for the future, we can assume that the vehicles in this box will be purely electric, and won't need the kind of ventilation that most of today's vehicles do.

6. On top of that box, build a basement, perhaps with alleyway access on both sides (these should be narrow - no wider than approximately 15 feet.)

7. Now we're on the street level, above ground - a building should rise up minimally two to four stories from here. On both sides, narrow streets should be constructed, reserved only for foot traffic. Shops, bars and restaurants of varying sizes should line both sides of the street.

8. Install vibration and light sensors inside each partition wall in the new buildings, as well as in the existing buildings that line the street. Establish a legal framework allows anyone to use the spaces inside each building for any purpose, so long as both parties on either side of each wall and floor agree to specified limits (with the existing owner's terms taking precedence over that of the newcomer - note that there is give and take here, as a space in which you can't make any noise or let any light leak out isn't worth much compared to one in which you can.) With this in place, inflexible one-size-fits-all zoning restrictions are no longer needed.

This arrangement can be implemented slowly, on a block-by-block basis, to transform any any every pedestrian-unfriendly American city into a vibrant urban paradise for people like me that seek this.

[lcr]

If you were to assume the soil you are excavating is very forgiving and your street is wide enough, you might be able to dig a trench that is 10 meters wide and 30 meters deep (you cannot dig straight down or the walls might cave in, unless you plan on using shotcrete, which you may or may not have to remove when you start backfilling the area). Now, we are talking about removing 975 cubic meters per meter of trench, if your street is 100 meters long, that is 97500 cubic meters you will need to excavate, at a rate of 33.1 cubic meters per hour, you would need to work around the clock for more than 122 days in order to excavate the trench.

You also have to take into account that you will need a place to store the 97500 cubic meters of soil you just excavated, which now have swollen up to 126800 cubic meters, the average dump truck can hold around 10 cubic meters, it would take you more than 10000 trips to move the soil.

This is just one street, never mind the fact that in order for this to be useful you will need to have at least a few kilometers of tunnel at a time.

Also, even if there are no internal combustion engines running in the tunnel, you still need ventilation shafts, subterranean metros have them for a myriad of reasons, even though there are no dinosaur squeezings being burned in the tunnels, for example, what happens if one of those very powerful batteries that run your Tesla happen to catch fire? People might suffocate if you can't force air into the tunnel.

Its a nice idea, but there is a reason why tunnel boring machines and vertical shaft sinking machines are so widely used on built-up areas.

[QuarterReptile]

This is precisely the type of thing that US cities need to stop doing. Building to a top level of service from the outset is a terrible plan.

What US cities need is a chaotic process of applying improvements, then making more permanent and high quality iterations as the initial fixes take hold. If your town has wide roads and cars that drive too fast, the logical step isn't installing curbs for protected bike lanes; it's paint to artificially narrow the road.

It won't give a perfectly clean implementation for every project, but it will prevent cities from making enormous bets that end up failing.

[AnimalMuppet]

It also leaves you a one-block-long tunnel for a train, and another for a road, for years until you get the blocks around it done - probably for decades until you get a system that goes anywhere.

[maxharris]

Sure, but you have to start somewhere. Why waste money digging up the same street over and over, as most cities do? It seems more efficient to do it once and be done for a long time thereafter.

[WalterBright]

Here in Seattle, we spend $$$$ buying a tunnel boring machine, bore a transportation tunnel for a section, and then ...

... cut up the boring machine for scrap!

It's like setting up a giant printing press to print a newspaper, and then printing 10 copies.

[Tomte]

At least you're getting something back. When the channel between Dover and Calais was bored, one of the boring machines was simply left underground, because it would have been to expensive to retrieve it.

[maxharris]

Also, you could easily imagine doing this to a part of the block that's up on a hill - Seattle and SF have a lot of areas like this. It would make piping existing traffic into and out of those transit sections easy.

[wrinkl3]

Has anything resembling that ever been implemented on a wide scale?

[maxharris]

Alas, not yet, no. (But that alone isn't a good reason not to try.)

[sillysaurus3]

Remember to leave room for alleys. NYC has no alleys, which is kind of a pain. Also buildings should be able to be built up to 20 stories, not 4. Your city will be popular and you want housing to be cheaper than SF to compete.