How grit works

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 does not melt snow straight away. It needs to mix with the snow to form a salty solution that helps it melt. When snow is expected, we spread grit in advance so it can start working as soon as snow falls, helping to reduce build-up and stop ice forming.

During heavy snowfall, snow can fall faster than the grit can work, so it may build up. In these cases, snow has to be cleared from the roads using a plough. Gritting in advance makes this easier by stopping it freezing to the surface. Spreading grit after snow has already settled has limited effect.

Best time to spread grit

When the forecast clearly shows roads will need gritting, we try to do this in the evening (around 7pm) or early morning (around 4am), before traffic builds up. This helps the grit work better. It also means roads are treated before busy times and before frost or ice forms, and when there is enough traffic to help spread the grit and mix it with moisture.

If the forecast is less certain, we monitor conditions closely and only grit if needed. We avoid rush hours where possible, as gritters can get stuck in traffic and delayed. Problems can occur if rain continues close to freezing, or turns to snow. In these cases, gritters must wait until the rain stops, 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.