New winter guide hits the streets
Five new chapters from the key guidance for winter service have been published by the National Winter Service Research Group (NWSRG).
The updated sections now cover the following core areas:
► Salt storage
► Treatment methods and technologies
► Spreader management
► Spread rates
► Treatment for snow and ice
The NWSRG said its guide is ‘considered to constitute the best way of providing national best practice guidance on these issues’. The latest version of the spread rates guidance ‘contains significant updates’, determined on the basis of research carried out by the NWSRG, TRL, Highways England and Transport Scotland over a number of years.
‘These are intended to both simplify the process of decision-making and allow for greater economies and efficiencies, particularly in conditions between 0°C and -4°C,’ the NWSRG said. ‘Three separate matrices are provided for dry salting, treated salting and pre-wetted salting operations, and relate to dry/damp and wet road conditions for surface temperature ranges down to -15°C,’ the guide states.
‘Based on research, it is considered that -15ºC is the lowest temperature at which salt without special additives can be considered as a practicable and effective de-icer on the road surface. However, it should be noted that salt becomes less effective when spread in conditions where the surface temperature is below -7°C (-5°C in ‘low humidity’ situations), and it is recommended that, whenever practicable, authorities avoid spreading salt in these conditions and utilise alternative deicers in these situations when temperatures are very low.’ Guidance for direct liquid application (DLA) treatments in UK conditions is currently still being developed.
Taking a risk-based approach
In line with the new code of practice for highways maintenance – Well-managed Highway Infrastructure – the guide advises authorities to take a ‘risk-based approach to the development of their precautionary salting spread rate schedules or matrices’ with reference to neighbouring authorities. This should take account of national guidance but also all of the relevant important ‘local climatic, geographic, network composition, resourcing and administrative factors’, as well as local experience of maintaining the road network.
The inclusion of ‘resourcing’ factors in the new guide, while in line with the new code, may raise legal issues when it comes to defending a strategy. NWSRG secretary and technical adviser Adrian Runacres told Highways: ‘Courts will be looking at the prioritisation principles in authorities’ policy documents. Decisions for winter service must be consistent with the council’s top level priorities in its policy document.’ Mr Runacres also explained that the underlying science behind winter service had not changed but the document provided a greater emphasis on practicability, using a new approach and language designed to make it easier to apply. ‘If you look at the spread rates, the numbers are the same but we have different bandings so there are fewer degrees of change, which makes it easier to use,’ he told Highways.
The guidance advises that the following factors can help determine the most appropriate spread rates for precautionary salting operations:
► The salting technology utilised, i.e. dry, pre-wetted or treated salting
► Type and condition of the salt, including grading and moisture content
► Performance and serviceability of the spreader, including whether the spreader is calibrated for the salt being used and if the spreader is within calibration at the time of spreading
► Road surface temperature throughout the period under consideration
► The amount of liquid water present at the time of spreading and during the following period
► Traffic levels on the network before, duringand after treatment
It categorises traffic levels into light, medium, high and congested – these relate to those anticipated around the time of the precautionary salting operation and ‘are not the same as the traffic categories generally used for other highway maintenance purposes’. Road materials can also impact winter service decisions. The guide reads: ‘Research has shown that, in winter conditions, porous asphalt can attain temperatures up to 2°C lower than denser surfaces such as hot rolled asphalt (HRA) and, in some areas that cannot drain freely, porous asphalt tends to remain wetter for longer. This increases the risk of ice formation on porous asphalt and consequently
more salt is needed to keep porous asphalt free from ice.’ It adds that: ‘Concrete roads can exhibit different thermal properties to flexible pavements and will tend to retain heat in their core for longer than HRA roads due to their denser construction.’ The guidance contained is not prescriptive and is designed to assist authorities in taking a risk-based approach to the determination of appropriate precautionary salt-spread rates. However, authorities should keep full and accurate records of their winter service activities, and where authorities choose precautionary salting strategies that vary from the guide, it is especially important to document risk assessment processes and the reasons for adopting different rates.
Getting the numbers right
One area where documentation is crucial is when authorities attempt to take residual salt levels into account, as this is ‘notoriously’ difficult to measure and the subject of ongoing research. According to the guide, when attempting to take such levels into account authorities should consider: ‘The precise circumstances when they will do so, as well as the information sources they will rely upon to ascertain, confirm and monitor residual salt levels on their networks. For example, these could potentially include: direct network observations; available data from road weather stations; the length of time since previous treatments took place, and the weather and
traffic conditions that have occurred since that time; coupled with the local experience of authority staff.’
Mr Runacres revealed that the NWSRG is currently putting a bid together to the UK Roads Board for funding this financial year for research into residual salt levels. The first stage of research would involve looking into what authorities can do to collate the data streams mentioned above, to produce the most accurate residual salt readings based on the information available. Industry is carrying out research and development on future technological solutions, but Mr Runacres said the NWSRG was keen to provide assistance in the short-term to help authorities with the data streams they currently have. ‘When we speak to members and authorities about the key priorities for ongoing research, lots of them say residual salt. We are trying to find some funding to work on this and
it’s consistent with work Highways England is doing. It’s a relatively high priority for them too,’ he explained. After the publication of Well-managed Highway Infrastructure in 2016, national winter service guidance was effectively transferred to the NWSRG’s Practical Guide. The new code no longer provides detailed guidance to practitioners regarding the delivery of the winter service and so the UK Roads Board, on behalf of the UK Roads Liaison Group, asked the NWSRG to make its Practical Guide generally available. The guidance has also been developed in association with the Institute of Highway Engineers. As work in each subject area is completed, a new section is made available.