Suitability
Salt works most effectively on well used roads as the tyre motion plays a key role in the process. It is a better management of limited resource to target such roads.
Availability
There is not enough rock salt produced to salt every road in this country. This became apparent in 2009-10 when there was pressure put on national salt supplies and central government took control of the distribution of supplies.
All local authorities were instructed to cut down the number of roads that were treated and Buckinghamshire reduced the salting network to emergency or 'snow' routes.
You might think that as the UK is surrounded by salt water, we would be able to extract salt from the sea to use, however Britain is too cold. We do not have the ambient temperatures required to naturally dry salt like the Middle East. The energy costs involved in drying salt any other way make such an option economically unviable. Importing this kind of salt from other countries costs twice as much as that of salt mined in the UK. It can only be justified in emergency situations.
Environment
Salting 40% of the road network does not have a significant impact on the local environment, however salting 100% of the network would see saline seepage into water courses at more harmful levels.
Criteria
Roads are assessed against a set of criteria. The resulting 'score' needs to be high enough for the road to qualify for salting. This criteria can be found here.