Summary

Main findings

In 2021, Scotland’s largest source of net emissions was from domestic transport at 10.9 MtCO2e, which excluded International Aviation and Shipping (source: Scottish Greenhouse Gas Statistics 2021). This represents 26% of total net greenhouse gas emissions allocated to Scotland. The Scottish Government is therefore keen to explore carbon saving opportunities associated with the transport sector. This desktop study searched for information on rolling resistance and how it affects vehicle fuel consumption and CO2 emissions. The main findings of the literature review are as follows:

  • 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 during the rotation of a wheel.
  • The coefficient of rolling resistance (Cr) provides an indication of how great the rolling resistance is for a given weight between the wheel and the surface upon which it is rolling. Determining the value Cr is complex, particularly in a road environment where the properties of the touching surfaces are constantly changing.
  • The measurement of rolling resistance is difficult and requires the use of specialist equipment that needs to be operated by skilled and experienced staff. Measurement methods identified, include the following:
    • Laboratory drum method;
    • Trailer method;
    • Coast-down method; and
    • Fuel consumption method.
  • A review of papers on measurement methods suggested that they are still in their infancy and under 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, i.e. 50 mm < wavelengths < 500 mm. A study that collected data from various trials that investigated the influence of rolling resistance on fuel consumption tentatively concluded that the average influence was 10% for macrotexture, 12% for megatexture, and 8% for unevenness.
  • Concrete pavements are likely to provide lower rolling resistance than asphalt pavements owing to their high stiffness. However, one study concluded that optimizing the texture of the surface would be more cost-beneficial than reconstructing pavements to increase stiffness.
  • 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.
  • Most of the studies use a simple formula to express the relationship between fuel consumption and rolling resistance. The formula assumes that rolling resistance accounts for about one‑quarter of a vehicle’s fuel consumption.
  • Studies in the USA estimate that the fuel required to overcome rolling resistance varies dependent on the vehicle type, drivetrain and weight and can vary between 4% to 33%.
  • One socio-economic study estimated savings in fuel consumption over 15 years as large as the entire cost of maintaining the pavement. The study modelled savings in rolling friction using reduced texture (MPD) and roughness (IRI) values. 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 microstructure, a fine balance needs to be struck to ensure that pavements are safe for all road users, particularly under wet conditions.

Recommendations

The literature review shows that it is desirable to specify roads that have a low rolling resistance. Road pavements that possess a low rolling resistance will result in a lower fuel consumption, with a corresponding decrease in GHG emissions. The review highlighted 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. As stated above, measuring the rolling resistance coefficient (Cr) is challenging but it is clear that it is related to the smoothness of a pavement. The latter is also 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.

With a view to reducing the rolling resistance of roads the following recommendations are made:

  • In association with the road industry, explore how materials in combination with construction and maintenance techniques could be used to optimize the profile of the road surface, particularly in terms of the level of macrotexture, megatexture and unevenness.
  • A road trial should be considered to:
    • Investigate the potential to create a smooth profile utilising low rolling resistance materials and machine control systems, e.g. stringless 3D paver control.
    • Study the texture wavelengths that improve ride quality and consider measuring Cr based on the trailer method.
    • It is important to monitor the trials to determine any degradation in profile (or Cr) as this is required for future modelling purposes.