Hmm I have seen this on many of the lorries and it says ...
SOUND HORN OK
My guess is that as they have less visibility then if you honk/sound your horn then they will know that something is going on behind them and they can take action on it and not cross a lane or perhaps slowdown a bit ... just my guess of course.
Hey Mats, Yes, You are thinking correctly. purpose of the phrase is to alert a driver of a vehicle approaching from behind to sound their horn in case they wish to overtake etc.
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