While (as a European) I agree with your point regarding speed signs, I think an important point is that there are plenty of traffic laws that differs (even) between EU countries. So when going driving in a foreign country one really should look up at least basic rules beforehand, and speed limits will most likely be front and center.
Note that if you cross a border in Europe (not necessarily in the EU) by car, you will pass a sign [0] giving the default speed limits for each type of road.
