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)|