This is getting to be a nightmare! If we have a sticker for every European country, including ones like Belgium, where the have different ones, for different area's, then the right hand side of the windscreen ( where they must be attached ), will be full of stickers.
They have different colours, for different countries/area's, so how can anyone accurately check, from the roadside, on a moving vehicle?
As I've mentioned before, my concern is spotting when I can or cannot enter a LEZ, because many are NOT fixed, but change, due to the pollution level, on a particular day. There is enough to keep track of, on the roads, anyway, without this additional issue.
I've currently got stickers for Germany, and France, both different colours and numbers, so there isn't any standardisation.