I don't understand how this is hacking-resistant; if the car is relying on external information, then any flaw in its authentication of the external information can be exploited, regardless of any changes deployed by the city.
The utilitarian decision is still to upgrade everyone, because the people who would hack cars to kill would likely do it another way. This may become the new improved way to "cut brakes." A state mandated system could upgrade hacking charges to serious felonies, which deters most except state-sponsored and psychopaths. I have a feeling that real car hacks will be as rare as poison Halloween candy
Also in a situation where someone is hacked, having a dense recorded model of all the roads in a city would be very handy for investigation, since the hacker would likely have to be near the car.