Winter Gritting: How Rock Salt works
Rock salt is used for treating roads, footways and cycle tracks and although it has a gritty appearance it is not "grit".
The salt works by lowering the freezing point of moisture on the surface. It is less effective at temperatures below minus five degrees centigrade (-5°C) and has virtually no effect below (minus) - 10°C.
The white appearance of a well salted, dry surface should not be mistaken for frost and it is normally safe. Often, even when dry, residual salt will itself attract moisture and can give the surface a damp appearance, but the salt should prevent the moisture from freezing.
There is no such thing as an absolutely safe surface.
The presence of salt will not always guarantee an ice-free surface so concentration and care is always necessary.