Isn’t most traffic caused by cascading human error? I assumed true FSD would be a mesh net where every car broadcasts its next move to its neighbors.
Only if humans are banished altogether from the roads. In which case the current attempts at FSD are pointless anyway and we should focus on building the infrastructure for simpler automated cars. As it stands today, every current automated car in existence is quite a lot more timid than a typical human and will cause gridlock very quickly as a result. An awful lot of driving relies on behavior assumptions that computers aren't good at making yet.
The NHTSA has been working on vehicle-to-vehicle communication for some time but we're still many years away from having it widely implemented.