Oh yes, definitely, on a road sign, the shorter the better:
Closed - structurally unstable
But the original sign wasn't short, which is probably a worse problem than the greengrocer's apostrophe.
In fact, since you have to pause and consider what "structural instability" means in real terms, maybe "Weak bridge" or "Subsidence", depending on which section is actually at risk, might be better, if less precise.