[Wistar]

Many modern cars have traffic sign recognition systems that extract the speed limit. The BMW system is fairly amazing to watch work as the speed limit display changes juuuust as you pass the sign and are surprisingly good at reading signs that are even partially obscured. Although I can't remember exactly which cars feed the detected speed limit into their adaptive cruise control systems, I think some of the Range Rover models do. Some even allow you to set a percentage or absolute speed value above or below the speed limit that you want the car to travel.

Others use a speed limit database and GPS/accelerometer system to establish the speed limit.

[SOLAR_FIELDS]

When I was driving in the EU I had a car that used gps to pull the limit (but never actually enforced that you followed it). It was really cool to hop on the Autobahn and see a symbol for none pop up in the indicator. It looked like a nonsmoking sign without the cigarette in it.