If you’re curious, this is a demo/experiment. The long term goal is tunnels so inexpensive that they can go 30 levels deep, letting us travel within cities at 200kph with no stop signs (even eliminate automobiles from the surface of cities)
This will require considerable progress in tunneling r&d, which is their primary activity
IMHO Tunnels for bicycles make a ton of sense - similarly to EVs low ventilation requirements, small diameter means cheaper build and most importantly protects you from weather elements.
Tunnels can recoup their cost easily if they are used massively by many people every day. Any reason why you are against building a subway / underground railway?
This will require considerable progress in tunneling r&d, which is their primary activity