How salt works

Salting does not mean that the road surface will instantly become ice free. Salt works by turning the ice or snow surrounding each salt granule into a saline solution which has a lower freezing point than water.

The action of traffic helps the salt to be more effective by moving the salt around and eventually melting all the ice or preventing ice forming.

Salt is less effective on road surface temperatures of less than 5°C and so will take longer to melt snow in these conditions. It also melts snow less than 40mm deep and only when traffic moves salt around.

Water freezes at 0°C - the presence of the salt prevents water from freezing until -6°C to -8°C. However, salt starts to become less effective at -5°C and almost ineffective at lower temperatures. In extremely low temperatures, or heavy snowfall, a mix of salt and grit may be used to help vehicles get about.

Why the road could still be icy

• Rock salt needs vehicles to drive over it to work effectively. Vehicles grind the salt into smaller particles to spread it across the road - this means that grit is sometimes not effective when there isn't much traffic or when there is a lot of snow. The more traffic a road has, the quicker the salt will take effect.
• Rain can wash salt off roads leaving them prone to re-icing. If the rain turns to snow during rush hour, any earlier treatment will be washed away and it isn't possible to re-salt in heavy traffic.
• If road temperatures fall below minus 7 degrees Celsius the effectiveness of salt diminishes and it will not prevent roads from icing up.
• If conditions are really bad, access to the roads is not always possible - even for the salt spreaders.
• Most minor roads and pavements are not routinely salted.

Why the gritters sometimes don't spread salt

Gritting vehicles have become more sophisticated, and lorries now dispense the required amount of salt directly down on to the road in a fine spray that you may not see. Treatments are occasionally treated pre-wet (salt and brine mixed) and are not clearly visible.

However, sometimes a vehicle may not be spreading any salt. This might be because:

• it hasn't reached the starting point of its treatment route
• it is returning to the depot to refill
• it is driving on a road that is not on the gritting route.
• it is driving over a section of road that has already been treated by a fellow driver

Every gritting vehicle is fitted with a GPS system which tracks its route, speed and location. It is part of the inspector's job to monitor this data to make sure the lorries don't deviate and their routes are being treated correctly.

If a salt spreader was travelling too fast to be safe

Contact the team and we will investigate your concerns. When spreading, salt spreaders should travel at 34mph maximum.