I think one of the major causes for the high cost of tunneling is that it is done too seldom. Any major city in the world probably has room for 10 or so additio

I think one of the major causes for the high cost of tunneling is that it is done too seldom. Any major city in the world probably has room for 10 or so additional tunnels, so why do they not operate a fleet of TBMs constantly for the next n years?