Water reaches its highest density at 4 degrees celcius that is 39.2 Farenheit. If the ground is cold at that time, like permafrost or a bridge etc. the water can easily crystallize & freeze.
Generally speaking, when it butt freezing cold, you know it! When you are driving a whole day from a warmer climate and you get into a sub 4 degree zone, the little warning can save your life as it may make you drive a little more carefully.
I think it is activated fom 39.2 to 32 degrees....
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Malcolm Forbes
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