[mncharity]

You might look at Elon Musk's The Boring Company. It's based on the ideas that self-driving electric vehicles permit smaller and simpler (and thus cheaper) single-lane highway tunnels, with simpler cheaper surface access, constructed rapidly at large scale (and thus cheaper), using mainstream vehicle tech, in a dedicated environment (permitting full automation, greater-than-highway speed, and vehicle coordination), woven in spacious 3D underground, with blended surface operations to whatever extent driverless driving becomes available. And thus traffic volumes/access/costs competitive with highways, buses, and subways. Perhaps picture something vaguely like Tesla passenger vans doing 100+ mph under bus routes, with van-is-the-cab elevators or ramps in parking lots.