Fine, good idea but.... the technology isn't good enough and probably won't be in time for implementation.
If the proposed technology relies on recognising signs, one kid with a paint brush could create havoc. If it is GPS based, our satnav shows speed limits and is usually accurate, but sometimes throws a wobbly as it passes under bridges or goes along side a minor road on a motorway. (this of course relies on Mr Trump not turing off or 'fuzzing', the GPS signals as they have been known to do in times of national emergency.
Imagine 1: doing 70 and passing under a 30 mph road.... car slams on the the brakes to stay legal, and a multiple pile up ensues.
Imagine 2: 3 lane motorway with artic in lane 1 doing 55.5mph, 3 artics in lane 2 doing 56mph and little you in lane 3 doing 70 mph, no acceleration available to get past quickly to clear the lane for the blue flashing lights behind you.
Feel free to let your imagination add further scenarios.
As the limiter will be computer controlled there will be an industry in hacking to remove the limiter. Recent air crashes seem to show that computer control isn't all it should be.