On a heavily-used block with tall buildings on both sides, here's a recipe that will work well:<p>1. Dig up the street and sidewalks, going down 60-100 feet.<p>2. At the bottom of this pit, drive in piles if you haven't yet hit bedrock.<p>3. Build a box, about twenty feet high, to run plumbing, power internet, and other utilities<p>4. On top of that box, build another one for a train.<p>5. On top of that box, build another one for vehicles to pass through. Because we're aiming for the future, we can assume that the vehicles in this box will be purely electric, and won't need the kind of ventilation that most of today's vehicles do.<p>6. On top of that box, build a basement, perhaps with alleyway access on both sides (these should be narrow - no wider than approximately 15 feet.)<p>7. Now we're on the street level, above ground - a building should rise up minimally two to four stories from here. On both sides, narrow streets should be constructed, reserved only for foot traffic. Shops, bars and restaurants of varying sizes should line both sides of the street.<p>8. Install vibration and light sensors inside each partition wall in the new buildings, as well as in the existing buildings that line the street. Establish a legal framework allows anyone to use the spaces inside each building for any purpose, so long as both parties on either side of each wall and floor agree to specified limits (with the existing owner's terms taking precedence over that of the newcomer - note that there is give and take here, as a space in which you can't make any noise or let any light leak out isn't worth much compared to one in which you can.) With this in place, inflexible one-size-fits-all zoning restrictions are no longer needed.<p>This arrangement can be implemented slowly, on a block-by-block basis, to transform any any every pedestrian-unfriendly American city into a vibrant urban paradise for people like me that seek this.