I think it's the combination of mandatory updates and zero tolerance for error that will be interesting.

For example city X decides to change some traffic rule on date Y. Each autonomous vehicle will need to receive an update by date Y. What happens if a vehicle hasn't been updated soon enough?

One solution is a mandatory update scheme. Every day a vehicle checks if it has received its daily update, if not the vehicle is disabled. Another more long term solution could be close co-ordination with cities to determine which upcoming traffic rule changes warrant mandatory (vehicle cannot be enabled without update) updates.

These corner cases may seem trivial, but with zero tolerance for error across hundreds of thousands of cities careful strategies will need to be developed.