There is a functional limit for induced demand! But it effectively can't be reached in urban and suburban areas because cars scale really poorly. Cars don't scale well for three main reasons:

(1) Cost. Building roads and highways inflicts both huge upfront capital costs and maintenance costs. A typical highway line has a throughput of 2,000 cars per hour. (2) Real Estate. Building the level of roads necessary to meet induced demand+ would take up an extremely large amount of land that can only be used for vehicles. (3) Local Roads. Local roads can only absorb so much capacity and they have functional throughput limits. If you wanted to make them faster, homeowners couldn't have driveways.

If you really wanted to, you could possibly make a city where driving is king and there is effectively no congestion. It would just be absurdly expensive and hostile to everyone who doesn't have access to car.

> you could possibly make a city where driving is king and there is effectively no congestion. It would just be absurdly expensive and hostile to everyone who doesn't have access to car.

This is exactly what has been done in most of North America, except for the "no congestion" part.

Only cost is important for tunnels, because there's still space sufficient for hundreds of layers of tunnels below cities.

Only if you build tunnels that no one can get to. Road tunnels still need to obey grade and (horizontal and vertical) curvature restrictions. For example, every story down you push a tunnel, every city block longer an access ramp needs to be. And the deeper you make tunnels, the more difficult ventilation and pumping becomes.

theoretically one could avoid vertical curvature issues by connecting the tunnels with large shafts, similar to elevators in skyscrapers.

This may slow down traffic, based on the vertical speed of the elevators and how quickly people in cars can board the elevator.

I'm not sure which one would be quicker: having a really long access ramp (long because of the tunnel's depth), or having to wait in line to board an elevator that moves quite slowly.

Musk's system uses elevators.

Now figure out how to get people up and down, and how to cool them. Places like London has problems with both already.

Look at Paris, this is not true.

In discussions of supply and demand, something that isn't often discussed is the real, physical limits of supply creation in terms of construction time, costs, manpower and such. Demand is more flexible.

This reality is also frequently ignored in the housing conversation when people advocate the solution to high prices is to simply add more supply, as if it's a given that supply could be created fast enough to overtake demand. I find that no one really ever does the work, taking into account the friction of real life on supply creation, to figure out if that's likely to be the case.

The limit is very high because it is not just the current pooulation, but all the new housing that is also induced. Where people live is a functon of transportation.