Streaming doesn't work. You are probably thinking about Microsoft flight sim, which completely fails (and is the first "completely streamed" map).

Streaming might be the wrong word. I'm talking about loading just enough data to do what's relevant right now. And I'm not talking about full 3D geometry of the world, that's not helpful to a vehicle. It needs to know in 2D where it can and cannot drive, as well as real non-static/dynamic obstacles (what the cameras and LIDAR are for).

You don't need gigabytes of data to load a 2D geometry of a small area like a part of a city, surely. You can also cache it on disk. Your phone can even do this. In fact, you can ask it to cache however wide of an area as you'd like. You might hit several hundred megabytes but that's like a whole midsize city probably.