Start by creating an elevated tram system that services 80%+ of the population. Envision something like the rail-based individual PeopleMover cars at Disneyland. For those unfamiliar with the PeopleMover, picture a small train car able to hold six people at most. If these were independent of one another, they could be called up using an app on your phone and take you to major shopping or event locations.

This reduces road congestion and frees up real estate. It also reduces the amount of data an FSD system has to deal with. You can now concentrate on using fully autonomous cars that travel the roads servicing the remaining 20% as well as transporting people to and from PeopleMover stations.