Yeah, the idea with a road sign is to convey info. in just a few words, as people are driving past. Sometimes a 2-sentence explanation isn't a possibility. But I think the idea of the sign (assuming it's not a photoshop job) is that large quantities of water accumulate on the road during rain, quantities so significant that it can effect how much control one might have over their car. Might effect steering or even stall out your engine if it's deep enough, who knows...