https://www.autoweek.com/news/a61532276/mandatory-speed-limiters-europe-cars/
TL;DR Cars have been able to figure out when they're speeding for a while, thanks to GPS as well as traffic sign recognition, and they've also been able to pump the brakes automatically when needed.
Having a computer automatically slow down a car in response to posted speed limits, therefore, was not really a question of technical feasibility for some time—but mandating it has been a question of political will.
That political will has materialized in the European Union, and starting July 7 all new cars sold in the EU will feature intelligent speed assistance (ISA) systems.