In Portugal when approaching a 50km/h limt they have a very civilised system. There is a marker for 70km/h limit first then about 100m further on is a marker for 50km/h at the entrance to villages/towns/built up areas next to the road etc. If your speed is above the limit there is a set of lights shortly after the 50km/h marker which will turn red for about 20 seconds. This gives you time to think about how naughty you have been and then the lights turn green again and you can ride away suitably chastened. It really is a great country.