My guess is more than 50% in the Netherlands and less than a third elsewhere in Europe. Wind has in common with hills that you have to work harder, but for some reason shifting down in a head wind doesn't really help as much as on a hill. It's sometimes also about other things, Flanders and North Germany is pretty flat too but legislature is national. If I'm not mistaken in Belgium there was a law that a bike had to have two brakes, which probably makes sense in the Walloon part that is quite hilly. So despite the cycling isn't that different, the Flemish are more used to a different type of bike.