GPS is helpful, and key, to autonomous cars. But let's not forget the various ways GPS can be hacked. We need multiple methods of navigation (visual/radar/lidar to stay within lines and avoid obstacles, inertial/GPS like aircraft use to sanity check your inputs, ground-based authenticated systems to identify current location (imagine being able to poll an intersection for its identity, we already have computers at every intersection with traffic lights)).

If we rely too strongly on GPS, we eliminate many existing roadways with tunnels, and many convenient constructs like parking decks, or new tunnels where they make sense.

GPS is more key for long term navigation and knowing when the car should look for a turn. Simple road following through LIDAR and other local sensors is enough for 90-99% of tunnels today since they are mostly just single roads from point A to B without required turns.

I mean GPS repeaters are a thing, but there's even better options, you could install small radio beacons in the tunnels and triangulate using those. Much more precise, and cheaper than GPS repeaters.

I'm sorry: what is it about cars underground that is a terrific idea when it costs more than the moon to build a tunnel these days?

I think he means more like madrid for instance. All the traffic is underneath the walkway level in the downtown area. It is really great to not have any cars. Because of this they have room for giant plazas everywhere and it makes for a much more pleasant experience.

GPS repeaters have existed for at least a decade.

Interesting, I didn't know that was a thing.

Cars and tunnels existed for a lot longer than GPS.

Yes, and no one wants to go back to the pre-GPS days.

I got the map out the last time I was in the car. Far quicker than waiting for my phone to get a GPS lock.

I don't even want to know what kind of phone you have if unfolding a map, tracing and memorizing your route, and folding it back up is quicker than a GPS lock. On any phone I've had since 2009, GPS lock has been instant.

All I needed to do was look up the number of a junction. The rest of the route was pretty straightforward. And it was not a foldable map, just a big book. Way less faff than getting the phone out of my pocket, unlocking the screen farting about looking for a map app, waiting for it to load then get a GPS lock.