Anyway, what's wrong with this sentence fragment? Well, "road liable to flooding" is grammatically equivalent to saying, "If this rain persists, the road is likely to flooding." Even a child would tell you that that is terrible grammar. The road is likely to flood, certainly but to say that a road is likely to flooding makes no sense. "Likely to" and "liable to" have to be followed by an infinitive. There are no two ways about it.
So, what could the genius that phrased this sign (that we in Ireland no doubt blithely copied verbatim without proof-reading it) have put? Take your pick of:
Road liable to flood
Road likely to flood
Road prone to flooding
Road subject to flooding.
And indeed, many more.
You'd think something as important as the phrasing on road signs would be handled by a department of people that can actually use the language properly.
