My local council (UK) are rolling it out on a town-by-town basis over the next 3 years. I'm pretty sure they'd do it quicker if they had the finances to buy that many new road signs all at once. It does mean the process is a bit confusing, since traditionally there are other visual cues for what a road's speed limit is, which generally mean even if you don't see the signs, you still know what the limit is. Now those visual cues aren't consistent between towns (two roads with the same visual cues, such as "street lights in an urban area", have different speed limits depending on which town you go to.) Since I live equidistant between two very similar towns that now have different speed limit rules... I wish the council had just changed all the speed limits at once!