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.