My car shows the speed limit (and other road signs) by detecting them from the signs. If you put it into cruise control, it will go at whatever the limit is.
Which is great. Except you get bits, (the most common one for me being Pimbo interchange on the M58) where there is a slip road or other road parallel to the motorway with its own speed limit signs. Whenever I go past the section, I could be doing 70, and the car sees the adjacent 40 and slams on the brakes! Now I know either to turn it off on that bit, or be prepared for it to happen and put my foot down to countermand the brake application. But the technology isn't quite where it needs to be yet.
(Same thing happens, but worse, with a "crash detection" system, which you can't turn off. If it thinks you will crash into something, it sounds a very loud beep, flashes a warning sign on the dash and starts to perform an emergency stop! Problem is, this can happen when you are on a sharp bend, and the car doesn't realise you will be steering, but assumes you are going straight ahead into whatever obstacle is there. It doesn't happen that often, but when it does, the combination of factors can be a bit of a shock, and distracting)