I think, Trainman, that your observation is the most practical way forward if control of vehicles were to be taken out of the hands of drivers in pursuance of obedience to the speed limits of the roads. As you say, tamperproof measures with severe fines/mandatory scrappage would be needed to deter maverick drivers. All the technology exists for linking GPS and engine management systems - firm commitment and action between vehicle manufacturers and government is the next step for implementation. And, because GPS is global, any vehicle subjected to legislation within its own country of location would also be under control when taken abroad.
On motorways, or any other roads for that matter, individual roads could have their maximum possible speed adjusted electronically through the GPS system, to accommodate weather, road works, or any situation that demands an immediate change of the limit - short or long term.
There would, of course, be the objectors; those who would make the "nanny state" accusations, although I don't know what logical grounds they could use to claim that drivers should not be made to obey speed limits.