Executive summary

The Scottish Government has set climate change ambitions to become a net zero greenhouse gas emitting nation by 2045. In 2021, Scotland’s largest source of net emissions was from domestic transport and this sector is regarded to be a key area where carbon emissions can be reduced. There has been an increased interest worldwide to understand how rolling resistance, the interaction between vehicle tyres and the road pavement, affects carbon emissions. WSP were commissioned by the Scottish Road Research Board to undertake an international literature study of published work relating to rolling resistance. The literature review is intended to be part of a scoping exercise to assess the potential benefits of specifying a pavement with low rolling resistance and to inform future research needs.

The literature review provides some background to rolling resistance or rolling friction and how it is currently measured. The study then reviews research that focusses on how road properties affect rolling resistance; the development of low rolling resistance materials; carbon savings related to low rolling resistance, safety concerns and particulate emissions. The research is discussed and the main findings are summarised. Some of the principal findings included the following:

  • The primary causes of rolling resistance are thought to be associated with deformations of the tyre and pavement. These deformations result in energy losses that are dissipated in the form of heat.
  • Determining the coefficient of rolling resistance (Cr ) is both complex and challenging, particularly in a live road environment where the properties of the touching surfaces are constantly changing.
  • A review of papers on measurement methods suggests that they are still in their infancy and require development. Standards exist for testing tyres but they are viewed to have serious drawbacks as they do not consider the fact that rolling resistance is dependent on a tyre-road interaction.
  • It is often difficult to separate the influences of the tyre and the pavement, but the relative contribution of the tyre to reducing rolling resistance appears to be more significant than what can be achieved through changes to the road pavement.
  • The main surface-related factor that influenced Cr in a major study was surface profile irregularities in a wavelength range between macrotexture and unevenness, namely megatexture.
  • One of the main factors affecting rolling resistance is the ambient air temperature as it affects both the tyres and pavement; the presence of water or snow on the surface increases rolling resistance.
  • Trials to develop mixtures that reduce rolling resistance have shown that they can reduce rolling resistance by around 5 to 6%, which roughly translates to 1.5% saving in fuel consumption. However, further monitoring is required to validate results and better understand the degradation of Cr in service.
  • One socio-economic study estimated savings in fuel consumption over 15 years as large as the entire cost of maintaining the pavement. However, other studies cast caution on this type of approach as it is very sensitive to the level of pavement condition and its deterioration rate over time.
  • Road trials in Denmark highlight that low rolling resistance mixtures can be more slippery in the beginning compared to standard materials and they need to be monitored to ensure adequate friction is being provided.
  • Research into particulate emissions indicate that tyre wear is of particular concern and work is ongoing to reduce wear rates and the toxicity of components that go into tyre construction.
  • From a pavement perspective, tyre wear is affected by the level of microtexture provided by the surface aggregate. Although there may be scope to reduce the level of microtexture, a fine balance needs to be struck to ensure that pavements are safe for all road users, particularly under wet conditions.

The literature review shows that it is desirable to specify roads that have a low rolling resistance, but that the properties of pavement are likely to play a relatively minor role when compared to other initiatives such as new vehicle propulsion systems and advances in tyre technology. Measuring the rolling resistance is challenging but it is clear that it is related to the smoothness of a pavement. The latter is likely to bring additional benefits, such as: improved longevity or durability; less wear and tear on vehicles; reduced maintenance costs; and provide a more comfortable ride for road users. Finally, the study makes some recommendations with a view to addressing gaps in research and reducing the rolling resistance of roads.