You think, in america, that someone would pay for putting all of our roads underground? No. They simply want to put a very tiny number of roads underground because even a catastrophically expensive tunneling project is cheaper than negotiating with all the people who own the actual land you would rather pave a road or rail through.
It really depends on the legal framework governing land use, e.g. do cities have the right to use the land beneath properties. And of course how affordable tunneling becomes.
