This is FANTASTIC! It's important to understand how massive the Earth is (even though it's a pinprick when compared to the sun). You can EASILY go 3-D or deep.
The deepest mines in the world are around 12,000 feet deep. This is about 6x the height of the tallest building in the world (2000 feet tall).
Imagine, thus, putting in a tunnel at what would be every 5th floor of a skyscraper and that you could fit 6 skyscrapers underground...that allows for approximately 150 stacked tunnels! Excessive of course but possible!
When imagining a 150 way merge I can't help but feel like it'll pollute enough to make nearby trees crumble in a matter of days. 150 ways at approximately 1 car per second (a very full highway) and 1.1 people per car (I hardly know anyone who carpools), there's 1501.1120 = ~20 000 people every two minutes. Trains can operate at the same frequency, carry a few thousand each and pollute a lot less -- heck, electric ones can draw from solar panels, reducing pollution to near zero. Give it ten tracks and you have equivalent capacity to 150 stinky lanes.
Musk's idea was that gas-powered cars would not be driving through tunnels themselves, but on electric sleds, which makes the tunnels as non-stinky as with electric trains.
I think it's rather scary how thin the mantle is in many places. The oceanic crust is just above 10km deep, I'm sitting on something like 15km. That's.. not a lot. I can cycle that in less than an hour.
The deepest mines in the world are around 12,000 feet deep. This is about 6x the height of the tallest building in the world (2000 feet tall).
Imagine, thus, putting in a tunnel at what would be every 5th floor of a skyscraper and that you could fit 6 skyscrapers underground...that allows for approximately 150 stacked tunnels! Excessive of course but possible!