If you read the comments that follow the article at the link below, they indicate that it may be an April Fool’s joke. It’s hard to tell for sure though because I’ve definitely seen stranger things and all transportation agencies are working hard to promote safety currently.
The article claims that pavements across Britain will be painted with speed limit signs that are visible only when it rains to warn people to slow down on the slick surface. Here’s the article if you’d like to check it out.