Many Norwegian cyclists complain about the amount of salt used to keep pedestrian and cycle paths free of snow and ice in winter. Snow- and ice-free asphalt reduces the risk of skidding, but salt causes bike frames to rust and degrades asphalt and concrete surfaces. Salt also has a negative impact on surrounding soils, vegetation and watercourses. Soils lose their fertility, and water masses are subject to oxygen depletion at depth.
On the other hand, paths that are salted rather than gritted generate less particulate dust in spring.
But the big question remains. Is it possible to stop using salt and still encourage more people to walk and cycle in winter? As part of the Norwegian Public Roads Agency’s R&D programme called ‘Bevegelse’ (Movement), we at SINTEF have conducted interviews with developers and made observations of pedestrian and cycle paths treated to comply with a variety of winter maintenance standards.
Salt or grit?
The general aim of getting more people out of their cars and walking or cycling all year round has led the Public Roads Agency to implement its two highest winter road maintenance standards on pedestrian and cycle paths in several urban areas in Norway.
The first of these is the ‘bare asphalt’ standard, under which pedestrian and cycle paths must be entirely free of snow and ice in the period from 6 a.m. to 11 p.m. It is permitted to use salt on such stretches of path in combination with snow ploughing and sweeping. No more than two hours must pass between any given action (ploughing, sweeping or salting) during the period when it is necessary to keep the path clear. Salt ensures that snow does not stick to the asphalt surface and prevents any water from freezing. The bare asphalt standard is implemented mainly in coastal towns with heavy pedestrian and cycle traffic.
The second, ‘winter road’, standard permits a path to be covered by a hard and even surface, or sole, of snow and ice. Under this standard, grit is used to provide adequate friction, and no more than three hours must pass before essential actions (gritting, sweeping or ploughing) are implemented.
A pedestrian and cycle path at Moholt in Trondheim cleared in compliance with the ‘bare asphalt’ standard. Photo: Hampus Karlsson
A pedestrian and cycle path at Moholt cleared in compliance with the ‘winter road’ standard. Photo: Hampus Karlsson
Just as good without salt?
SINTEF has been comparing conditions on stretches of path in Trondheim, cleared in compliance with the two standards, under different weather conditions. We studied pedestrian and cycle paths in the districts of Moholt and Lade during the winter 2019/2020.
Along most stretches of path cleared according to the bare asphalt standard, we found that clearance was in compliance, and that the path would be seen by most people as safe and attractive for walking or cycling (see the first photo).
These results were not surprising, but the question remains: can we achieve conditions perceived as safe and attractive without using salt? Several of our observations indicate that this is in fact the case. The second photo shows that clearing a path in compliance with the winter road standard also offers people a safe and firm surface on which to walk or cycle.
About the project:
The aim of the R&D programme Bevegelse (Movement) is to generate new knowledge about society’s needs in relation to the effective maintenance of pedestrian and cycle paths, forms of collaboration, contract design and the supervision of contractors.
The risk of ice and slush
However, under coastal climatic conditions, as is the case in Trondheim, the weather can change rapidly, with frequent fluctuations of air temperature above and below freezing point. This creates particular problems for paths cleared in accordance with the winter road standard, because the snow and ice sole may deteriorate due to cycles of melting and re-freezing in response to temperature fluctuations, resulting in an irregular ice-covered surface. We observed several cases of such conditions during our inspections. In situations where a snow sole had previously been established in compliance with the winter road standard, we encountered slush on the paths that could easily have been removed by ploughing.
A drawback of the bare asphalt standard is that melted water can seep onto the asphalt from the banks of cleared snow along the sides of the path, and then freeze to form ice as the surface becomes colder. When this happens, a large amount of salt will be required to keep the paths free of ice and snow. This situation is less of an issue on paths cleared in accordance with the winter road standard because here it will be possible to use grit to achieve the stipulated levels of friction. Another disadvantage of the bare asphalt standard comes to light in locations where implementation of the two standards meet. Here, the use of salt only serves to destroy the snow sole when it encroaches onto stretches cleared in compliance with the winter road standard. This situation can create difficult and dangerous conditions, especially for cyclists.
Implement adequate procedures so that both standards can be effective
Based on our observations, as well as interviews with developers, we can conclude that both standards can be effective in most weather conditions. We only observed poor compliance with the standards in a very few cases, and in these situations, the cause was not the choice of maintenance standard, but a lack of systematic implementation. The key success factor for the winter road standard is to ensure adequate procedures for identifying the need for patch clearing actions. Actions should not only be taken in the event of precipitation, but also in response to changes in the weather. This will ensure that slush is kept to a minimum and that the snow sole remains firm and even, especially when re-freezing occurs.
The bare asphalt standard will continue to offer benefits that are difficult to match using the winter road standard, in spite of the negative impacts of salt. First and foremost, it offers an entirely regular surface with good friction, which is essential for people with reduced mobility, as well as for wheelchair and rollator users and those pushing baby carriages. Another advantage is that there is no need for gritting, resulting in less particulate material having to be cleared away in spring. Cyclists, in turn, will then have no need to worry about roadholding or punctures caused by grit particles.
The right standard at the right location
The results from our follow-up studies indicate that both standards have their benefits and drawbacks. Both can play a role in persuading more people to walk or ride their bikes in winter. However, more consideration than is currently the case should be given to exactly where the different standards are implemented. For this reason, the Public Roads Agency has launched a project designed to revise the criteria used to select either the bare asphalt or winter road standard under a given set of conditions. For example, the impact of salt on the environment will become a more crucial factor.
Another of our findings is that the most important issue is not the selection of the standard, but the correct implementation of the path maintenance actions that accompany each standard. This was also the conclusion of another report prepared as part of the Bevegelse R&D programme; ‘Betydningen av drift og vedlikehold for gående og syklende. En kunnskapsoppsummering’ (The significance of winter path maintenance for pedestrians and cyclists – a knowledge summary).
Predictability is key
Winter path maintenance is a very important issue if Norwegian towns and cities are to achieve their targets of persuading more people to walk or cycle throughout the year. Predictability and day-to-day consistency in implementation of the standards is essential if people are to leave their cars at home. At the same time, we must recognise that our urban environments are complex systems with multiple factors to take into account. A better set of guidelines for the best use of the existing standards will thus be a useful contribution. In the longer term, this will ensure more effective maintenance of the greater roads network.
This article was first published on the website Forskning.no on 25 May 2021.
