The cost is calculated in many millions of pounds. To reduce these costs, winter maintenance teams keep important roads free of ice and snow. They use sophisticated sensors and weather forecasts, but their most important tool is salt, applied to the roads by the familiar salt spreader, the 'gritter'. The use of salt for winter maintenance depends on the fact that dissolving one substance in another alters the freezing point of the second.
Salt lowers the freezing point of water.
The greater the concentration of salt, the lower the freezing point, although there is a limit of temperature below which salt will not melt ice. This is well below usual winter temperatures in the British Isles. Melting begins on contact when salt is spread on ice or snow. This produces water, dissolving more salt and producing further melting. Heat is absorbed in the melting process - the latent heat of fusion.