New surface routes have large costs (in real terms, to say nothing of money) because they must displace whatever is there now. Tunnels, of course, avoid this cost, and also avoid noise along rights of way, scars through ecosytems, many potential accidents, and so on.

But (as usual), this is out of reach because of cost, which is to say the cost of things, which is to say, in large measure, the cost of making things. We're still very bad at that.

(Gee, I could repeat this in so many contexts...)

Words and ideas I offer here may be used freely and without attribution.