Why is salt used on icy roads?
Salt does not necessarily melt ice on the road, but this is why you will see gritters applying salt and grit before the cold weather actually hits so that the water mixes with the salt.
Saltwater has a far lower freezing point than pure water so when placed on slush or prior to snow, there is a decreased chance of the roads becoming icy. Salt is combined with grit in order to prevent freezing as well as create some friction for the tyres on the road.
Ice forms when pure water (rainwater) reaches zero degrees celcius; however when you add salt so it becomes a 10% salt solution, the required temperature for freezing drops to -6 degrees celcius and a 20% solution will freeze at -15 degrees celcius. As salt is so abundant and many countries do not actually (or rarely do so) reach temperatures below zero degrees celcius, it is a logical choice for improving road safety and improving driving conditions in wintery weather.
If the temperature is already at zero or freezing, then the salt would be ineffective, so gritters will prioritise their timing schedules for applying ice in different regions and countries. However, if the temperature is above freezing, the salt will have an effect as it will be able to sit in the grooves of the ice and in turn, help to soften the ice.
This softened ice will tend to turn to slush, which can still present a potential problem for drivers on the road, however, when applied in conjunction with the grit, it provides a safer and more responsive surface for driving on, which will then help to decrease accidents.
An icy road will present a problem as the ice greatly decreases any friction on the road, so that steering is less responsive, braking distances are vastly increased (and if there any kinds of inclines, it could even be impossible to stop the car properly) and the chances of accidents are greatly increased.
Black ice in particular presents numerous problems as it is practically invisible to the human eye and as drivers may not be expecting the roads to be icy, the chances of accidents occurring are higher.
It is imperative that councils apply salt and grit to roads in wintery conditions in order to protect drivers and minimise the chances of any accidents; some areas will also operate a snow-plough service when the snow build-up is too high for gritting and salt to be effective alone.