The salt is hygroscopic which means that it attracts moisture from the air and it only becomes activated when it is 'trafficked'. When the grit is driven over the salt grinds down and mixes with the moisture it attracts and becomes the ideal brine, a salty water, which is when it is most effective.
Similar to how local authorities manage gritting, the Estates department will grit all University of Kent roads and main footpaths before starting on other areas across the campuses. For more information see the University's Snow and Ice Policy
Learn about snow clearing.