When it snows (in the other world, not in this South African one), the authorities put salt on the roads. This lowers the melting point of the ice and snow, meaning that it melts even if the temperature drops below zero. Thus, it has to be colder for there to be ice and snow on the roads, meaning that generally, there will be less ice and snow on the roads throughout the winter. Thus, fewer accidents. Or so the logic goes, anyway.
In the cities of the USA, about 19.5 million tonnes of salt is spread across roads in this way each year, and a lot of it comes from a dried-up 400 million year old sea about 2000ft below Ohio. There are some photos and some stories of the mining process on Wired and they’re rather interesting:
Apparently, when anything breaks down, deep underground, it’s simply discarded there. Why waste time, energy and money in getting it back up to the surface just to throw it away anyway?
I foresee some extremely confused archaeologists somewhere way down the line.
Tenuous a-ha link: Name of salt mine in question is Morton. Oh, and effect of the salt is surely to Stay On These Roads, ne?