Cyclists stay back! stickers

I am a bit confused by the stickers. Is there a measured danger in drafting such vehicles, as they imply? I suppose an emergency stop could lead to a nasty bump but it doesn't seem like a lethal risk.

It's more about cyclists undertaking(more so at junctions). When all the traffic is stopped some will scoot up the side of a HGV and then the lights change and the HGV makes turn left with a chance they haven't seen the cyclist it can all get messy.

Like a more cycle specific one of these.

Problems I can think of are when they just get plastered on any/all on the road it doesn't signify a difference(so if all cars had long signs on it would make the sign irrelevant).

Your post does nicely sum up how unclear/badly worded the sign is, I'm sure many share in that not knowing.