DULUTH, MN (KDAL) - The Minnesota Department of Transportation is advising drivers to slow down as slippery conditions have been reported on northeast Minnesota roads.  These cold temperatures are also ideal for the formation of "black ice".  The so-called "black ice" is formed when warm and cold air meet and is most prevelent at intersections where exhaust condenses into water droplets that freeze on the pavement.  The nearly invisible ice also tends to form on bridge decks, ramps and in shady areas.  MnDot also reminds motorists that salt is only minimally effective when temperatures are below zero.