I guess one possible way to make a tunnel is by box-jacking, which is a new way to build tunnels in days (not months/years):
I only found one company that specializes in this in the United States (though this technique seems to be much more common in the rest of the world). Plus, I haven’t seen any examples by them for tunnels smaller than multi-lane roads.
With that said, if Kane and the City think it’s a good idea, maybe they could use this technology, and use the extra tunnel space for underground “outdoor” seating or pop-up stores/events (and use construction space for additional buildings)? Perhaps something like this (jacking could be done in the yellow arrow’s direction)…
…or this?
Note that blue arrows are entrances/exits to existing parking decks. This means the southern route (1st image) is more likely to get in the way of increased drivers, unlike the northern route (2nd pic). Plus, the former requires a North State Bank branch to be torn down. I’m not sure if that’s worth it, though, since the latter idea could be more convenient depending on where the planned North Hills bus transit center gets placed. (Note that bridges will also have the same problem where it could easily look awkward and out of place. To design a better sense of space, maybe tunnels aren’t a bad idea, after all?)
Either way, if NCDOT is the only problem, I think it’s time our civil engineers got creative. Whether we end up using modular bridges or this fancy digging technique, it looks like North Hills could be a good test bed for these techniques. If they go well, maybe it’ll make it easier to bring them into DTR as well, if/when they become necessary?