you could also put in a subway track down the middle while you're at it, but the primary point of undergrounding the road is so we get our most precious above g

you could also put in a subway track down the middle while you're at it, but the primary point of undergrounding the road is so we get our most precious above ground space back for people, not cars. it's not simply about optimizing throughput, though that's also a worthy, and compatible, goal.