Earlier quoted context omitted.
How about if cities were built in such a way that you would just have buildings and podium being pedestrian and bike friendly with all transportation network being 1 level underground (or 2 levels) all as self driving EV pods. This may increase the no of possible 'roads' and does not sound as far fetched in terms of overall costs. Emergency vehicles can be given an exemption to operate on the podium
Tunneling underground is extremely expensive. Individual self driving pods aka cars are very expensive. But most importantly cars do not get more efficient in capacity just because you put them underground, plus now you have to build entrances and exits to the underground system everywhere. Traveling also becomes a lot less appealing when you're in a black tube all the time rather than seeing the city around you. It'…
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.