Gritting does not mean that the road surface will instantly become ice free. Grit works by turning the ice or snow surrounding each grit granule into a saline solution which has a lower freezing point than water. The action of traffic helps the grit to be more effective by moving the grit around and eventually melting all the ice or preventing ice forming.

Grit 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 grit prevents water from freezing until -6°C to -8°C. However, grit 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

• Grit needs vehicles to drive over it to work effectively. Vehicles grind the grit 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 grit will take effect.
• Rain can wash grit 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-grit in heavy traffic.
• If road temperatures fall below minus 7 degrees Celsius the effectiveness of grit 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 our gritting crews.

Does grit melt snow

No. Grit doesn't directly melt snow as it firstly has to mix with the snow to form a saline solution and lower the melting point. If snow is predicted, grit is spread in advance so when the first snow falls it can start to mix with grit to create a saline solution which can reduce the build-up of snow and prevent the formation of ice. However, in prolonged periods of snowfall the snow can fall at a rate faster than the grit can mix with the snow which means the snow may accumulate. The accumulated snow will have to be ploughed away from the roads, but this is made much easier by grit spread in advance of the snowfall as the grit already applied reduces the likelihood of the snow freezing on the surface. Placing grit on top of snow which has already fallen has limited benefits.

Best time to spread grit

This depends on a variety of factors. When we know with certainly from the weather forecast the roads will require gritting treatment, where possible we carry out the gritting treatment in the evening at approximately 7pm and / or in the morning at approximately 4am prior to commuting traffic building up on our roads. Gritting at this time ensures the best performance of the grit. Spreading grit at these times also means the roads are treated in advance of peak traffic flows, before the times where frost and ice will form, and when there will be enough traffic on the roads to help the grit mix with the moisture to form a saline solution.

Where the weather forecast cannot confirm with certainty that a treatment will be needed, we will continuously monitor the weather data and order a grit treatment only if it is needed. If the time of gritting coincides with the rush hours, the gritters can be severely delayed and get stuck in traffic, so we always try to avoid gritting during rush hours wherever possible. Difficulties can arise when rain is forecast to continue right up to the time of freezing or when the rain is forecast to turn to snow. In these circumstances the gritters must wait until the rain has stopped or the grit will be washed away.

Why the gritters sometimes don't spread grit

Gritting vehicles have become more sophisticated, and lorries now dispense the required amount of grit 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 grit. 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 gritter was travelling too fast to be safe

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