Strange thing, Jim, but I was about to mention a similar one on a road I used to work near. In that case, it was one of the mobile hazard warning signs like ones used for roadworks, which warned of ice in the middle of the Summer. It was a drainage ditch with a small stream running under the road, which flooded in heavy rain and iced the road in winter.
We tended to just return the sign to the Highways Department when the weater got hot.

As for fixed road signs, they say things such as "Scunthorpe welcomes careful drivers" I keep watching for the one saying "If you drive like a nutter - go to Hull"...

Quack!
GtD.