They come in various sorts, but they're all essentially giving the same message - don't try to pass this vehicle in the blind spot on the left (or occasionally at all), the driver can't see in that space so you're putting yourself at risk in doing so. And it sounds like eminently good advice if its an articulated lorry, but since this sticker appeared its finding its way onto smaller and smaller delivery vehicles which really oughtn't have sucky visibility.
Most would agree with the advice of not slipping down the side of large vehicles in that way, but I'm sure many of us would question the presence of such an advisory note on, say, a transit van. And while these things are getting ever more common it doesn't look like cyclists opinions are being sought over what this means for our safety - which is allegedly what they're for.
I'm not opposed to this advice - but I think it has to be two-way. If you put one of these signs on your vehicle you're admitting that such a vehicle isn't really very good for safe use in a multi-mode environment. I would argue that for a city such as Cambridge the presence of such a sticker tells us straight away that your vehicle needs to stay the hell out of the city centre, and I would urge our City and County Councils to discuss implementing this as a simple measure to make cycling safer. Urgently. These companies are telling us flat out that their vehicles are not safe in the presence of the majority vehicle type in the centre of Cambridge - what possible excuse can there be for bringing unsafe vehicles into the city centre?
Health and safety is more than how the hazards we pose should affect the behaviour of others around us - its more important, and more responsible, to limit to the hazards we ourselves pose by modifying our own behaviour. Does your vehicle have a massive blind spot, presenting such a hazard to others that you cannot safely be passed on the left (or the right)? Is that space so dangerous that a cyclist needs to be warned? And you coexist in city traffic with cyclists and may often end up slowly passing them, putting them in that (invisible) space for such an extended period of time that other road hazards may distract you from them? That isn't okay.
I propose that such a warning sign is primarily about the hazards posed by that vehicle, not the behaviour of others not in that vehicle. You want such a sign on your lorry? Great, thanks for the advice, but it means you can't overtake cyclists on any road where any congestion may lead to stoppages. I propose that such a sign is useful, but it is absurd to suggest that if putting a cyclist in your blind spot is dangerous that any driver of such a vehicle can reasonably choose to do so. Want a cyclists stay back sticker? Fine - the price is your vehicle isn't allowed to overtake cyclists in any non-national speed limit road. In a 20, 30 or 40mph zone especially you cannot overtake cyclists if your vehicle has this sticker - and if the purpose of such a warning is the safety of the cyclist, there is no arguable case against this proposal.