grammaticality - "Road liable to flooding" — is this roadsign grammatically correct?

I passed the roadsign below while driving home late last night, and realised that despite how many times I had seen it, I was still surprised by the choice of words used and unsure if it was actually grammatically correct.

After all, the wording on roadsigns is crucial. As we all know, it needs to be clear and succinct.

So, to my mind the words

Road liable to flooding

should be

Road liable to flood

or it could be rewritten

Road prone to flooding

Am I right, or is the original ok?

Answer

Road liable to flooding

The OED helpfully cites the above warning in its definition of liable:

[with infinitive] likely to do or to be something:
patients were liable to faint if they stood up too suddenly

(liable to) likely to experience (something undesirable):

areas liable to flooding

The word flooding is used here as a noun rather than the usual infinitive. The OED also supplies a similarly helpful entry for this form:

(as noun flooding)
a serious risk of flooding

In other words, it's perfectly fine!

I suspect that flooding is preferred to just flood to avoid the ambiguity over what is flooding, as the word is not used only in reference to water. "Road liable to flood" could mean that the road itself could increase "in overwhelming quantities".