In the last few weeks, I’ve noticed more and more of these cropping up in my contact’s Flickr photos:
They’re a modification of London’s standard signs, used to warn double decker drivers that they may want to avoid high branches. (I’ve been on plenty of buses that have run into them, making a disturbing but - usually not actually damaging - impact.)
They first started showing up (with a slightly different sticker design) in March 2010:
It seems they’re produced by an organisation called Climate Rush, and one of the nice things about them is that they’re subtle, and don’t actually stop the signs from performing their usual job. More like this.