Before and during adverse weather conditions, which are likely to prove hazardous to motorists and pedestrians, local highway authorities will often apply salt and gritting treatments to the road and pavement surface.

This section of the website aims to highlight the various ploughing and gritting measures, treatments and guidance which could be applied during periods of adverse weather.

Why and when should street authorities undertake preventative measures?

The main objective of preventive measures is to create road traffic conditions which do not take drivers unaware. Each county and town in the UK will experience different types and degrees of adverse weather conditions, which will all have varied impacts upon the movement of people and goods.

Severe Weather Plan

Each highway authority will establish their own Severe Weather Plan (SWP) to keep paved areas free from snow and ice, leading to a reduction in winter accidents; maintain mobility in winter weather, and; to fulfil the Secretary of State obligations as the Highway Authority. The SWP is the method by which the service provider describes the way in which the winter service requirements will be met. This should be reviewed and updated annually.  It should comprise different activities that combine to achieve the performance requirement. The SWP should describe the different activities to be undertaken as part of the winter service. Advice on the treatment of carriageways, footways, cycle tracks is embodied in the Highways Agency’s Network Management Manual and the SWP should reflect this advice in its development.

Salt and de-icing materials

One such preventative treatment is the spreading of salt on paved areas.  Salt is spread for the following reasons:

• To prevent ice from forming;
• To prevent snow from bonding to the surface, facilitating snow removal;
• To melt ice already formed;
• To melt residual snow after mechanical snow removal operations.

If snowfall is expected, the highway authority will usually spread 40g of salt per square metre on the roads before the snow comes.  Then as the snow falls, salting continues with snowplough attachments used if the snow accumulates. 

The substance spread on the roads is rock salt.  It is more or less the same as the rock salt you'd grind onto your food, other than it is brown due to containing an impurity of Marl, which is a type of clay. The salt is spread onto the road, but it works best when it goes into a solution.   It relies on the tyres of cars passing over the top of it to crush the salt onto the road. This then forms a solution with a higher de-icing capability. Salt melts the ice as it is mixed up by car tyres, creating a saline solution that disappears down drains, while grit blocks up drains. Water freezes at 0C, but the salt stops the water from freezing until -6C to -8C. Salt starts losing its effectiveness below 25F (-4C) and is ineffective at 14F (-10C). The majority of highway authorities use 6mm salt, which reduces the risk of damage to windscreens, though 10mm salt may be better for spreading wider roads and for treating settled snow. 

Maintaining salt supplies

Recent winters in the UK have seen several periods of prolonged snowfall, which has impacted upon the highway authority’s ability to maintain its salt supply. The House of Commons Transport Select Committee conducted an inquiry into the impact on transport of the winter weather in December 2010, which produced a series of recommendations. Specifically in relation to salt supplies, the inquiry found that strategic salt arrangements introduced in 2010 helped ensure that local authorities had sufficient salt to keep main roads open during a particularly severe winter. To this extent it was a considerable success.

The criticisms heard of the winter 2010/11 arrangements were largely points of detail which the Government is expected to consider in reviewing the arrangements ahead of the 2011/12 winter. In particular, the inquiry recommended that the Government consider ways of ensuring that strategic salt supply arrangements are more transparent to local authorities and that new guidance on salt spreading rates is followed. The Committee called on the Government to publish a written statement on the outcome of its review of the strategic salt arrangements before winter 2011 (HoC, 2011).

In response to this request, the Government stated that following the 2010/11 winter, the DfT has carried out a review exercise with the aim of identifying any lessons learned that can be taken forward to improve future winter crisis response mechanisms. For example, the DfT now have arrangements in place for regular monitoring of salt stocks to identify risks early on and take any further action where necessary, including providing advice to local highways authorities.  In advance of winter 2010 and during the winter itself the Government was proactive in its engagement with local authorities to ensure an effective emergency response to the ahead of and during the winter. This included a joint letter from the Secretaries of State for Transport and Communities and Local Government to each English local authority on 12 November 2010, stressing the importance of an effective winter service strategy and a further reminder in December 2010 of guidance produced by the Department with the UK Roads Liaison Group (UKRLG) on the range of actions that can be taken with regards to winter service and measures to ensure that salt stocks last longer.

Strategic salt arrangements – winter service 2011/12

Speaking at the Surveyor’s 20th annual winter service conference, Cold Comfort, on 27 September 2011, Minister for Local & Regional Transport, Norman Baker, gave a video address on the importance of keeping key transport infrastructure, like our road and motorway network, operating efficiently and safely as possible. A recording of this address, and transcript, is available on the DfT website and can be viewed by clicking here. Further details of salt arrangements for the winter service period will be available on this website as they emerge.

Standards for Highways: Routine Winter Service Code (RWSC)

Routine and winter service is the name for the work traditionally carried out under the name of routine and winter maintenance but is now better described as the provision of a service to the road users. The term ‘service’ also more adequately describes many of the activities traditionally referred to as routine maintenance. Elsewhere in the UK, the term "winter service" is also used and has been adopted for local roads in England. The name routine and winter service has therefore been adopted in the Code, which describes the requirements for routine and winter service activities on the trunk road network.  

The Code covers the Performance Requirements for highways and structures and the operational winter service on the motorway and trunk road network. It shall be noted that nothing in the Code shall relieve or absolve the Service Provider of any of its obligations to comply with legal and legislative requirements. The Service Provider is also reminded that compliance with all relevant standards and codes of practice forms an implicit and inherent part of the Code. Much of the work is covered by lump sum duties in many Managing Agent Contractor (MAC), Maintenance or Managing Agent (MA) and Term Maintenance Contractor (TMC) contracts. Where the routine and winter service activities are not covered by a lump sum payment, the Code should be read in conjunction with the NMM for bidding, allocation and outturn of funds for these activities (DfT, 2011).

Salt reserves and treatment strategies: ICE publications

The ICE has published several items on salt treatment strategies and applications. The most comprehensive, current and recommended of these publications is the ICE Manual of Highway Design and Management (2010). Chapter 61 of this publication is devoted to ‘winter service and other weather emergencies’ and provides an exceptional overview of the engineer’s role in the winter service. The Manual can be purchased from: www.icemanuals.com.

The ICE’s Design and Practice Guide for Highway Winter Maintenance (2000), provides good information on winter highway maintenance, with chapter 2 providing guidance on the importance of salt-stick management as an essential element of winter maintenance. The report can be downloaded by clicking here.

ICE Municipal Journal articles related to the topic of salt and gritting strategies include the briefing prepared by C. Cranston in 2004 titled: Winter road maintenance: to salt or not to salt?, which provides a case study of how Devon County Council found innovative ways of using technology to support its service delivery.