Northest: Discolored or Red Residue Driver Floor Mats

Question to all Prius Owners in the Northeast United States:

Why do most, if not all, cars from the Northeast have discolored, red residue, driver side floor mats, brake peddles, accelerators peddles, etc.?

Is it because of the snow, salt, or what?

Asking because all the cars that I have seen that come from the northeast all have the same pattern. I have never seen that I southern United States cars.