I worked for North American/Sifto Salt for 5 years and had my own salt business for 15 years. Many State/Cities use magnesium chloride, which is a by-product of the Salt/Potash solution mines in Salt Lake. Mag chloride in itself is much less corrosive than conventional sodium chloride road salt, how ever the mag chloride is very hydroscopic (loves to attract moisture down to 20% relative humidity, always stays wet!) The next issue is many Cities and local governments, parking lots still use sodium chloride. Vehicles, bridges infrastructure gets a bath of both chemicals resulting in a super corrosive mixture that keeps eating 24hrs a day and night even in a relatively dry conditions (even dry heated garages). Mag chloride also leaves a residue much more difficult to be washed off (possibly impurities in the mag chloride) by water from vehicles infrastructure so it has a longer corrosive season than regular salt alone.
