> cars do not get more efficient in capacity just because you put them underground

I don't think the entire "car tunnels" thing is reasonable either, but this one is wrong. The bottleneck for cars is the inherent interference in the 2-dimensional streets. If you shove them all in a few tunnels right until they get into a low-transit region close to the destination, it would increase their capacity by a lot.

It's also a huge amount of money that is better used some other way. But it has an effect.

Surface streets aren't inherently 2D, though - you can build overpasses and underpasses and above-grade highways. We just don't, because it's hideously expensive.