Correct me if I'm wrong but I think what is spread by 'gritters' these days is almost always rock salt. I believe this is fairly impure salt dug out of the ground that has a relatively small amount of sand mixed in with it. It lowers the freezing point of water so melts snow. For as long as it lies on the ground it will prevent re-freezing. As it disappears down drains it will need to be replaced. I don't think Councils use grit (or cat litter!) on roads and pavements because, unlike salt, it blocks drains. Salt is, of course, pretty bad for cars - although modern cars are much better protected than older ones were. Incidentally, salt is not used on airports because of the damage it does to aluminium planes - which is one reason why clearing airports is such a difficult job...