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I think what is happening is that the salt is drawing up all the shit that is embedded/soaked into the road surface like diesel, soot, oil etc and bringing it up to the surface where it get mixed together with the ice that didn't get a chance to form. A bit like greasy roads on a rainy day after a long dry spell. That's my two penn'orth.
Constable_Savage-
I think you might be right on the lifting of crap out of the surface. If they are putting a water disperser in the salt that would almost certainly contain or act as a detergent which would lift the oils and possibly even leech out some of the tar from the surface.
So many reports of slips this year I wonder if they have changed the formula, tho I had a big off last year that I still maintain was down to the post-salt pre-rain sludge.
middleofnowhere
I get the feeling that some of that goo is left over residue from the salting/gritting of the roads in the cold snap. I wonder if they put in some sort of oil based anti-caking agent that gets left behind as the temperatures rise.