Surely the way to manage this would be to make the local authority responsibe for providing real time speed limit data to autonomous cars... Some kind of transmitter in lamp posts or on existing speed signs.or even an API that can be called send your route and it returns current limits for the roads that your traveling on and near taking into account temp road works etc...
Having the cars recognise and read actual speed signs seems a bit stupid.
Biting the hand that feeds IT
