Under Ground Roads Everywhere

Imagine a sub division where all the streets were underground.  Your parking was below your house and on the surface yards were bigger, kids road safely wherever they wanted and instead of wide slabs of asphalt, trees and grass took up the space in front of your lot.  I know it would be expensive to start with and perhaps a sub division is a bad idea but think for a second about a place like Manhattan.  That place is worth a ton of money and every stinking inch of it is developed.  What if you could convert all the street space into extra buildings by burring all the traffic?  The space the roads take up could probably be sold to developers in order to pay for the tunnels.