La Defense and tunneled roads
Someone on Facebook mentioned La Defense, in Paris, as an example of all the roads in an area being tunneled. I can't find the link, because Facebook's search is terrible, so I'll reply via email.
I hadn't looked at La Defense in detail before, and it indeed does have more tunneled roads than other places I've seen. Therefore, my estimates of the feasibility of large-scale tunneling should necessarily update up. However, the update is fairly small, because there are quite a lot of asterisks attached.
- First, most obviously, La Defense doesn't actually have all the roads tunneled. Many are tunneled, but from satellite photos, there are also two surface highways, a surface ring road, and a number of smaller surface streets. I haven't yet found a clear map of the whole thing, but from rough guessing, it looks like about half the roads are tunneled as measured in lane-miles. Tunneling the second half would be more expensive and difficult than tunneling the first half, but not dramatically so, so it's true that it's at least plausible France could have made it entirely tunneled (if they happened to have enough money to burn after WWII).
- Second, La Defense is extraordinarily dense - from some back-of-the-envelope math, around ~400,000 people per square mile during working hours, comparable to midtown Manhattan (between ~34th and ~59th St.). That's ~40 times the density of Berkeley, and ~100 times denser than a typical American city like Houston or Phoenix. That density greatly reduces capital cost due to under-utilization. In a place like Houston, which is mostly detached single family housing, a road has to be built to every single house even though one house generates just a handful of trips per day. Roughly speaking, in a dense enough place, if one lane can hold 2,000 cars/hr. and peak demand is 500,000 car-miles/hr., you can build 250 lane-miles of road and still have every building next to a road. In less dense places, you might have to build 2,000 or even 10,000 lane-miles of road to reach every building, most of which will be vastly under capacity even at peak times.
- Third, La Defense has good connections to the Paris Metro (rapid transit), RER (commuter rail), Transilien (suburban rail) and a tram line on dedicated tracks, which are high-capacity and absorb the bulk of the peak time travel demand, unlike US cities where nearly all trips are by car.
- Fourth, La Defense has a near-total monopoly on tall buildings, not just in Paris but in the entire country of France. Per Wikipedia, 19 of the 26 tallest buildings in France are located there, which gives them monopoly pricing power over businesses that want large, urban offices.