0F is defined as the temperature that salt on ice reaches. Regular salt is used a lot in Minnesota because it works fine most of the time and is cheap. It doesn't work on the coldest days, so about 15F they start adding in salts other than NaCl. Below -15F they no longer have a salt that works at all - but those days are rare.

My Grandpa worked for the MN highway department until around 1995 when he retied, so my information is a bit out of date, but chemistry doesn't change that much so I doubt it is very different today.