How salt works
We use rock salt (grit) to treat the roads,
pedestrian areas and stock salt bins. This page has been populated
with frequently asked questions about how salt works and
how it keeps our roads safe.
We hope we're able to answer your question
here, but if you need more help please call us on 0845 121 2979 or
email winter.service@rochdale.gov.uk.
Why is salt used on roads and pedestrian areas?
Treatment of roads and pedestrian areas with salt is carried out
to prevent frost and ice forming and reduce the build up of snow to
help provide a safe passage on the highest priority roads and
pedestrian areas.
How does salt prevent frost and ice forming?
Frost and ice form when water freezes. Salt spread on roads and
pedestrian areas mixes with any moisture and creates a saline
solution. Saline solutions freeze at a lower temperature than
water, so frost and ice doesn't form on the road, even though
the temperature is below freezing for water. It's for exactly the
same reason why salty sea water freezes at a lower temperature than
fresh water. The actual freezing point of a saline solution depends
on the salinity (strength) of the saline solution.
The applicable amount of salt is spread on the roads to ensure,
as far as possible, that the salinity of any moisture on the roads
is sufficient enough to prevent the formation of frost and ice.
Will salt melt snow?
No. Salt 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, salt is spread in advance so when the
first snow falls it can start to mix with salt to create a saline
solution which can reduce the build up (accumulation) of snow and
prevent the formation of ice.
However in prolonged periods of snowfall the snow can fall at a
rate faster than the salt can mix with the snow which
means the snow may accumulate. Accumulated snow will have
to be ploughed away from the roads or cleared in the pedestrian
areas, but this is made much easier by salt spread in advance
of the snowfall as the salt already applied reduces the likelihood
of the snow freezing on the surface.
Placing salt on top of snow which has already fallen has limited
benefits. Ideally, snow should be cleared before salt is
applied to the road or pedestrian area.
When is the best time to spread salt?
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 6.30pm and /
or in the morning at approximately 4.30am. Gritting at this
time ensures the best performance of the grit. Spreading salt
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 salt 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 salt spreading 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 salt will
be washed away.
What is rock salt?
The salt we use on the roads and pedestrian areas is rock
salt. Rock salt is more than 90% pure Sodium Chloride (salt)
with the remaining insolubles consisting mainly of Keuper Marl
which helps to protect the salt quality. Marl also helps improve
friction when salt is used as a highway de-icer.
How long can rock salt be stored for?
We cover our salt stocks to ensure it is well preserved and
in the best condition for gritting. This is why we always ask
for all salt bin lids to be kept closed to prevent
the salt quality from deteriorating.
If rock salt is kept dry it can last for a long time. It is
also more effective if it is kept dry before it
is spread. If it's exposed to water it will dissolve and
will be washed away, meaning the salt concentration is less
when it is spread so it is less effective. Rock salt will also bind
together when it gets wet so has to be broken up before it can be
used. However, there are often clumps left, which means the rock
salt won’t spread as evenly and will be less effective as a
result.
Where does the council get its rock salt from?
Our salt supplier is Salt Union - the operators of the
UK's biggest rock salt mine at Winsford in Cheshire and Britain's
largest supplier of natural rock salt. Salt Union produces salt all
year round, which allows us to build our stock levels through
the spring, summer and autumn.
Facts about salt and treatments
- Road surface temperature and whether the road is wet or dry
determines what grit treatment is needed - not the air temperature.
Even on cold days the roads may have retained enough heat for
treatments not to be necessary.
- Over 2 million tonnes of salt is spread onto the UK roads each
year. The majority is spread onto motorways, trunk roads and main
roads. Less than a third of other roads are treated.
- Salting UK roads costs over £150 million per year.
- Without salting, delays caused would cost £2 billion per
year.
- Dependent solely on the weather, winter service expenditure can
vary vastly, making control of budgets very difficult.
- Each year, approximately 1,000 people in the UK are killed or
seriously injured on icy or snow covered roads.
- EuroRAP survey results show the risk
of being killed in a traffic accident in Britain is 5.9 per 100,000
population - the lowest in any country in Europe.