the construction of the second avenue subway was slow because of hundreds of years of infrastructural technical debt in manhattan and reduced tolerance for utility disruptions in an affluent area.
you can dig a hole (as elon musk has done) in nevada in the space of a few months.
We knew we needed to replace it in 1989 after Loma Prieta. In 2001 the Nisqually quake damaged it and made its replacement mandatory. It wasn't decided to build the tunnel until 2009, it then took until 2019 before it opened.
you can dig a hole (as elon musk has done) in nevada in the space of a few months.