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Page last updated at 13:21 GMT, Thursday, 3 January 2008

How do they know when to grit roads?

Gritter and a pile of grit

The Magazine answers...

When snow is forecast, local councils send out the gritters. Trouble spots are identified by networks of sensors embedded in the asphalt. How does this early warning system work?

On roads and highways across the UK, discs are embedded in the road surface to measure climatic conditions. Each is connected by cable or mobile phone technology to an automatic weather station, an unassuming grey box by the roadside.

Road sensor in Dudley (photo courtesy Dudley Council)
Sensors measure road and air temps, rain, dew and salt levels
GPS is now being used to provide detailed ice predictions
Authorities use data to decide when to send out gritters

It's a system developed in the 1970 and 80s and now widely used across the country to track and predict road conditions throughout inclement months. To have accurate information about driving conditions is invaluable to road authorities and local councils to decide when - and where - to send out the gritters.

The sensors are sited either on a representative stretch of road (no nearby trees, buildings or bridges, which offer some protection from the cold), or traditional cold spots. The weather stations then beam back data about air and road temperatures, wind speed and direction, and the wetness of roads.

This is posted on an intranet for officials to monitor, along with analysis by meteorologists using local weather forecasts.

Salt levels are also measured to ensure that grit already spread has not been blown away by wind or washed away by rain, says David Sparks, of the Local Government Association.

Automatic weather station linked to road sensors in Dudley
Sensors link to this automatic weather centre (grey box on right)
"What we don't want is a worst-case scenario such as we had in the Midlands a few years ago. Cold weather was forecast and the gritters went out. But then the weather changed. The snow turned to rain, which washed away the grit, then the temperatures suddenly dropped again and the rain froze on the roads.

"If you have a sensor measuring wind temperature, you'd know to send the gritters out again if it drops cold again."

Cold spots

With the UK's changeable weather, where sudden cold snaps can follow balmier days and vice versa, councils have gritters on standby from October to early April.

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At Dudley Metropolitan Borough Council in the heart of the Black Country, the council is responsible for keeping the roads clear in the city and the surrounding hilly district.

Its sensors are placed on the outskirts of the urban area and at traditional cold spots, and are linked to four weather stations throughout the borough.

"At times like this when the weather may change quite quickly, we're looking out for it 24 hours a day, and exchanging information with other authorities in the Midlands," says John Millar, the director of the urban environment.

Gritters load up before heading out on Northampton's roads in winter 2005
Gritters load up as forecasters predict snow and chills
The council has been using the sensors - which are constantly getting more sophisticated - for about eight years.

And new systems that use GPS technology are available, such as one developed at the University of Birmingham that combines geographical data from satellites with weather forecasts to provide more detailed ice predictions. The system is used by local councils such as East Sussex, Falkirk and Stirling, and the Highways Agency as part of a trial in northern England.

"It doesn't do away with sensors entirely as it's sensible to have eyes on the road to complement the forecasts in case of any hiccups," says Steve White of WSI, the company which bought the system from the university.


Below is a selection of your comments:

Why not make this information available to motorists? Imagine your satnav being able to warn you about icy roads on your journey. Even better to plot a less risky route or warn you to take care.
Mark, Nottingham

Mark, it already is. I get warnings come up on my TomTom as part of the traffic service
Pete, Chelmsford

These vital services are really appreciated, but they would be even better if the gritting on the pavements was more widespread, as it is often difficult to walk around in icy conditions, especially when walking down hills.
Karl Chads, London, UK

Agreed that there should be more grit on pavements, especially on hills. Last year I was a childminder during the winter and trying to push a heavy buggy up a slippery hill was boarding on impossible.
Lucy, London

I totally disagree with gritting the roads, Half the time the councils put it down when the roads are dry and there is no chance of rain, this then draws moisture out the ground and causes black ice, even if this doesn't happen, the salt generally makes the roads slippy and far more dangerous than a slight frost in the early morning. Also the corrosion it causes to cars and the road surface. If they are going to put it down then they should be made to clean up afterwards. I had a serious car crash a few years ago where a gritter had reached the county border and dumped a load of grit on a bend as he'd turned round to go back on his route.
David Tidy, Horley, Surrey

Mr Tidy seems to lack some of the basics when it comes to understanding the point of gritting roads. Rock salt when in solution with water will not allow it to freeze at the temperatures we are subject to. The gritters do an excellent job and save many lives with the work they do. I can live with washing my car a few extra times over the winter to get rid of the salt if it means the roads are safer.
Padraig Egan, Haddington, Scotland

I'm as yet unconvinced on the value of gritting. The major roads are a must but with the roads on estates being so windy the gritters cannot get down my street, and that is usually the hardest part of my trip. Also when you drive past a gritter in the opposite direction you're lovely paint job manages to get scratched by all the grit that seems to go everywhere but the road.
Stephen, Caerphilly

Why do we use salt at all? I lived I Sweden for a while and there they use small stones/sand mix, when it all melts they sweep it up ready for reuse next year, much kinder to the environment and probably cheaper too.
Graham W, Brighton, East Sussex

I was brought up in mid Wales and we own a house in Scotland. In both areas the grit can make the difference between being stranded, or worse, on a remote road and returning safely home.
Elli Carlisle, Royston

There is a lot of "overgritting" going on. Often the air is dry enough, so the road will not get slippery from a bit of frost. The grit also gets into the ground water and causes environmental damage. Alternatives, such as calcium magnesium acetate or magnesium chloride, are better but cost more. The grit itself causes a degree of slippiness, causing the road to be more dangerous, esp for scooters/motorbikes, and takes a lot of time to "rain off". More restraint is definitely called for.
Otto, Sunderland

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