I'm now travelling in France and - as any Italian - one of the first thing we learn when running on highways is that the road signs have a different colour.
In particular, in this case it is pretty difficult because higways are green in Italy and blue in France, while state highways are exactly the opposite (blue in Italy and green in France).
I browsed a bit arounf and find a nice page on wikipedia, which lists all (very many) roadsigns (not just highways).
There is also a reference to "European Standard for Traffic Signs - EN 12899-1:2001 Fixed, Vertical Road Traffic Signs – Part 1: Fixed Signs, Requirements" (couldn't find any link on the internet) which is expected to be the 2007 EU standard for roadsigns.
Therefore I'm still amazed by the fact that the European countries still have, in real world, so different signs (colours, dimension etc).
Does anybody knows if there is some work going on on this? Shall we ever have common EU roadsigns?