Or, just add speed bumps, curves, narrowings in the road, etc, to naturally bring the speed down. Then it works on everyone, whether they have the nanny tech or not, and you avoid the difficult problem of determining the speed limit accurately 100% of the time across every segment of road everywhere.

This is kind of like the (apocryphal) "NASA spent millions on a pen that would work without gravity; russians just used pencil" parable.