> That is, replace road lanes with something like tram tracks.

My brother, you are describing a train.

> I'm imagining vehicles being two/four person pods zipping around.

Oh, never mind. Yeah, the reason that doesn't work is that if you're going to spend the money to build tram tracks or zip lines or whatever predefined path, then it is only economical to make a few set predefined paths. If you make a predefined path for every possible commute in the city, you will run out of money before getting very far. So ultimately, I would say a train is the closest thing to your ideal.