Doing everything in real-time is difficult with current sensor technology and processing algorithms, and leaves a lot of room for error. The way that they're doing it currently is fairly clever for minimizing the amount of calculation/reaction time. I'm not so sure it absolutely needs to be real-time, so long as the system works.

That said, I'm no stranger to the 3D CAD universe, and I can tell you that housing all of the spatial data, regardless of format, is a LOT of disk space. A one inch resolution for 2,000 miles of mapped roadway is going to cost you.

The best solution might be to operate in "blocks" of spatial data, similar to Minecraft. If the car fails to load the next block, it alerts the occupant, who will be forced to take the reins, or else the car will pull itself off of the road and park near the horizon of its current block until a connection with the server is restored. Naturally, for your native city, you'd just have the whole thing stored locally. For road trips, input a destination and route to download into the car before you left. You might only need the blocks solution if you're driving aimlessly, or didn't have time to input destination/route.

You're always going to require a GPS connection or the next-generation equivalent sometime anyway.

Sorry, just playin' devil's advocate, like I do. :)