Autonomous vehicles can use existing roads more efficiently. In fact, that's the whole point of this tunneling project. Think of a tunnel as just a stupidly expensive road and you'll see that simply having autonomous vehicles without the billions of dollars of infrastructure would achieve the same goal. Simply close a few roads for normal traffic and you have the same advantages but without the tunnels and power rails. Vastly cheaper. And there already are lots of roads. So, these vehicles could go to a lot of places instead of only a select few elevators above the tunnels.

And of course you could actually mix autonomous and regular traffic as autonomous vehicles get smarter. Like is already happening in parts of the world.

Also, the main issue in cities is not tunneling into a central location but distributing goods all over the city using e.g. cargo bikes, small vans, etc. Many of those are already electric and typically operate from distribution centers on the edge of the cities. Banning regular traffic out of the cities would free a lot of space for that. Some cities are already doing that or at least strongly discouraging car traffic. E.g. London, Paris, Amsterdam, etc. have large car free zones and lots of modern electric delivery vehicles.