Ban personal self driving cars, then when the fleets outnumber personal non-self-driving cars, ban them too.
Presto...big brother now knows where you are, all the time. (Yes, I get that cell phones serve that purpose now, but the workaround is pretty easy)
I was assuming governments forcing all fleets to share data. Probably via some "safety / think of the children / OMG terrorists" bill.
Road usage configuration is something that external systems will already force self-driving cars to obey, a lot of bridges are 3 or 5 lanes with a middle that switches directions, or have some partitioned third lane group that switches similarly. Broadcasting that information over the web/whatever may help car co-ordination, especially it may help with route planning and traffic analysis, but these systems should be rather strongly isolated from the car itself and be offered from a source open to the general public.
There is a lot of interesting stuff here, especially when you get into traffic balancing, but none of it would need to pass through a car relay network or anything of the sort - there may be economy in doing so, but that sort of a propriety (or open yay) caching of state wouldn't impact or impose requirements on car-to-car communications.
As an aside, Federal Transportation guys don't bother invoking terrorism, they just go with "yeah these are two ton things moving around and we are the rule makers."