Why don't you use salt when it's really cold?
When temperatures fall to 10 to 20 degrees, the action of the salt (which lowers the freezing temperature of water) takes longer to work. Wet pavement refreezes at a much faster rate. The melting of ice and snow with the dilution of the salt between zero to 10 degrees accelerates refreezing even more. Continuing to apply salt at these very low temperatures will actually cause more problems than it solves.