Mostly Icy, Very Dicey

There is a chance, I suppose, that there are one or two people new to the state who might not suspect that heavy snowfall coupled with below freezing temperatures might add up to ice on the roadways...but fortunately for them the state put up these handy signs to alert them to that very fact.

Wonder how the drivers of yesteryear worked out what the road conditions would be like after a night of heavy snowfall and sub-freezing temps? Probably with some sort of ice divining rod or like primitive tool I suppose.