Hmm roaming costs you say. Well that is marginal problem with people crossing the boarder and can be addressed in ways any traveler knows already anyway.
The problem the GGP wanted to show is that there are always scenarios which could lead to the self driving vehicle to be confused - in your scenario with bad reception or lost connection for any other reason (aliens eating the antenna etc). This is what somebody pointed out already about airplanes - autonomous airplane control systems can let the pilot sleep most of the way after he programmed the target. The programming of the target may be wrong (in themiddle of Indian ocean?) or you end up in the airfield having no support for autonomous landing or it is out of service or whatever such thing. Good engineering practice and most likely also law will require to have a backup that can support such automatic system by not ideal but better than nothing system called human.
Unless of course we reach end of human development which is a completely different discussion but think of this - humans are last resort in such unpredictable situations because after appropriate training they are universal enough to realize what to do and give it a shot. System with such ability may be technologically possible one day but they would have enough of general human ability to adapt to environment that they would warrant some of our rights to be given to them. At the same time such systems will be complex enough that the probability of error will increase. The cost of such systems will increase too at least initially yet the problem with rights will have to be addressed - in other words at some point costs of enterprise will be not in technical ability but legal rights of the robots. Where this leads you can read up in Washing Machine Tragedy
. This story may be a joke but the problems of complex systems will at some point arrive. I for one cannot wait for a good washing machine - a self driving car I do not really need.