I suggested on the TFL lorry questionnaire that any vehicle that 'needed' a sticker on the back should also have one on the dashboard saying "remember, there may be cyclists or pedestrians near this vehicle" or similar. Both sides should be warned, that gets rid of the 'But I've got a sticker' syndrome.
I was following a flat bed thing today and the sticker was so small it was impossible to read until you were almost on top of it (lots of writing in the 'don't pass on the left hand side of this vehicle' vein) I was half expecting it to say 'if you can read this you're already too close'.