Behind the Forecast: How road salt actually works
Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
LOUISVILLE, Ky. (WAVE) - A snowy forecast means that road crews have to gear up for long hours and hard work.
The Kentucky Transportation Department (KYTC) is responsible for 63,000 lane miles of roads. Their fleet is made up of 1,024 plow trucks and four tow-plows. KYTC also has 400 contracted snow plow trucks to help with the main fleet.
The Indiana Department of Transportation (INDOT) has more than 1,000 snowplows and up to 2,000 drivers, mechanics, clerks, and managers working to keep roads clear. INDOT is responsible for 29,000 lane miles of interstate highways, U.S. routes, and state roads.
For both KYTC and INDOT, crews work 12-hour shifts to keep roads clear.
If the ground or a surface (maybe an overpass or bridge) is at 32°F or lower, snow, sleet, and freezing rain will form ice once it hits those surfaces.
Air and soil are fantastic insulators and terrible conductors; they don’t allow heat to transfer quickly or easily. Bridges and overpasses are typically made of concrete and steel; both are excellent heat conductors. Heat is lost quickly from the tops, bottoms, and exposed sides of bridges and overpasses when surrounded by cold air. Winter precipitation is more likely to stick on elevated surfaces because of this principle.
Salt works by lowering the freezing point of water which makes it harder for winter precipitation to stick to roads. It keeps water molecules from forming solid ice crystals. The salt must be in a solution with water for this to work; this is why brine is used.
Brine is usually applied to roads before winter weather occurs. Once the precipitation falls, it activates the residual salt. The brine keeps snow and ice from bonding to the roads; this makes roads easier to clear later on.
A 10-percent salt solution will freeze at 20°F while a 20-percent solution freezes at 2°F. The brine used on roads typically mixes with snow and ice, lowering the freezing temperature and melting the snow around it.
When pavement temperatures are at 20°F and rising, brine is used so hazards are avoided. Brine is an anti-icing initiative.
In lower temperatures, calcium chloride is added to salt to help melt the ice and snow. Salting is a de-icing initiative.
While KYTC uses sodium chloride, some other organizations around the country use other deicers. Calcium chloride, magnesium chloride, potassium acetate, and calcium magnesium acetate can also be used as deicers.
The salt used on roads works with the sun or friction of our cars driving over it to help to melt the ice into a slush which mixes with the salt; this keeps it all from refreezing.
The use of salt makes it easier for crews to plow everything off of our roads.
In 2020, the apparent consumption of salt in the United States was 53,000 metric tons, according to the United States Geological Survey. Forty-three percent of that salt was used for highway deicing, equalling 22,790 metric tons.
Sometimes sand is used in some locations. While it doesn’t lower the melting temperature, it does help to provide extra traction to reduce sliding.
Between 2007 and 2016, there were more than 1,235,000 weather-related crashes, according to the Federal Highway Administration. around 5,000 people were killed and more than 418,000 people were injured in those weather-related crashes during that time. Most weather-related crashes occurred during wet weather. A smaller amount happened during winter weather.
- 18% during snow or sleet
- 13% occur on icy pavement
- 16% of weather-related crashes take place on snowy or slushy pavement.
Icy pavement caused 156,164 crashes, injured 41,860 people, and killed 521 people between 2007 and 2016.
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