Engineering convention puts tunnelling as the most expensive way to build infrastructure - for example the Boston big dig cost 1 Billion dollars per mile - the kinds of costs only car drivers can ( and do ) consider.
Elon Musk claims he can cut the cost drastically - by speeding up the tunnelling process and reducing the cross section.
Normally I would dismiss tunnels for bikes as unstimulating, you have to go down and up - but they are dry and could go any where. Something perhaps to be part of a mix.
What do you think
