The gritters spread salt thinly across the road with a ready mixed salt and water (brine) solution. This allows the salt to start working as soon as it hits the road as it does not depend on vehicle motion to spread it and then turn it to brine. Mixing the salt with brine also means that less salt has to be used, meaning the salt supplies last longer.
Salt works by lowering the freezing point of any moisture that comes into contact with the road to -8ºC. If the road temperatures drop below this, the salt becomes ineffective and ice will begin to form.
The quantity and direction of the spreading can be adjusted, depending on the width of the road and predicted temperatures. It also allows the gritter to treat both sides of the road on just one pass.
Salt used to be mixed with molasses to help it stick to the road, however this was found to be the main cause of corrosion to other vehicles on the road. By removing this, the salt becomes far less corrosive.