Dogs shall be carried

I was in London recently, and while I was at King's Cross station I noticed that the escalators, which bear the usual safety warnings, now say among other things that 'Dogs shall be carried'. Maybe they've always said that, but it's more common to see the warning as 'Dogs must be carried'. This has given rise to the well-known joke, also seen in the recent Paddington film, in which a child is late for school and when asked why, he says that the sign said dogs must be carried and it took him ages to find a dog.

So my question is, does using 'shall' instead of 'must' remove this comedy interpretation or improve the sign in any way? No one really uses 'shall' any more, but it indicates future and apparently has some sense of being a command. 'Must' also expresses a command ('deontic necessity' - it can also express 'epistemic necessity' as in 'It must be cold outside'). Does 'shall' seem politer? Does a warning sign need to be polite? Unknown. Some more good suggestions here.

(photo credit not known; it's all over the internet)