these devices will probably be in operation as quick as the 'speed cameras in cat's eye and litter bins' scare.
Current infrastructure is not able to do this,
go past a services slip road on a motorway where the adjacent sign for that road says 30.
GPS? I bet we have all experienced driving in a field parallel to the road as indicated on the lost signal, not to mention going over road bridges where the road underneath has a different limit.
Smart motorways are already showing 40mph in a tailback where you can only do 10mph or stop/start, and 60 when you are barely able to do 25
Other than having a maximum limit of 70mph, how can they operate it safely otherwise. It will be an 'aid' for us to turn off/on rather than a full control device, and it will be used for prosecution/insurance to access after an event