This is called precautionary salting. A snow plough is normally used on snow more than an inch thick before salt is applied. Although salt can be corrosive to some metals and harm some plants, sensible precautions minimise damage. Modern cars are designed to resist corrosion. Street furniture and structures are well protected.
Salt causes no damage to road surfaces and roadside plants are selected to withstand exposure.
The most important way in which the environment is protected lies in the skill of the maintenance teams. Using accurate forecasting, they apply salt only when needed, using properly adjusted equipment to optimise the salt applied. Well-managed winter maintenance saves lives and keeps British business moving, with minimal impact on the environment.