"assuming the self driving cars don't do something stupid"
This is a big assumption.
This requires that all cars are self-driving cars capable of complex reasoning on in-car compute without relying on network connection, as network connections can't be assumed reliable in hurricane conditions.
Gov will always have access if they really want to, but my suggestion above prevents Flock and private companies from connecting vehicle location data to vehicle owner.
How about digital license plates?
That change the code displayed daily.
Not unlike authenticator phone apps.
Police can still use them to identify the vehicle, and verify registration, but mass surveillance and repo companies can't use them to track vehicles for more than a day OR to identify vehicles.