Climate change: evidence review of mitigation options in the Transport sector

Evidence review of potential climate change mitigation measures in the Transport sector.

7 Quantification of air quality, noise and congestion

7.1.1 Quantitative approaches

For a number of the options, the benefits in terms of congestion, air quality and noise reduction relate to modal shift from the car and other private transport modes. In terms of guidance Web-TAG and STAG provide information relating to impacts on air quality and noise, a summary of which is provided below.

For air quality an understanding of the impact in terms of:

  • Changes in passenger car unit/vehicle kilometres travelled by mode by study area
  • Changes in speed by mode by study area
  • Emission factors (in the context of the above)

is required, with a comparable approach used for Web-TAG (Marginal External Costs) guidance and Scottish Transport Appraisal Guidance ( STAG) (Technical database Section 7.43.). STAG suggests the use, where possible, of changes in the level of population exposed to the pollutants. Here the use of an emissions exposure estimate is suggested with reference to emissions and the population per zone. Associated spreadsheets for this analysis are provided. In terms of the economic valuation STAG highlights that air quality impacts should be calculated using a hybrid approach. This combines damage cost and marginal abatement cost ( MAC) methodologies.

For noise STAG highlights that the approach taken is different from that employed by WebTAG, with the DfT having recently published new guidance on noise, including monetisation. At the strategic level STAG suggests a qualitative treatment and a broad range of data sources relevant to the Scottish context are identified, including the use of land-use maps - location of areas which are particularly sensitive to noise e.g. schools and hospitals. At the project level, the emphasis is placed on the outcomes of the use of spatially detailed transport models which will allow an in-depth understanding of transport flows. WebTAG guidance focuses on the use of spatially detailed models. The importance of estimations of the affected population and monetary values are made with reference to Defra guidance, modelling tools and research ( Defra, 2014).

For congestion, reference is made in WebTAG but not STAG. For example, congestion is covered in A5.3 rail appraisal and A5.4 Marginal external costs.

In terms of models, there is comparatively limited number of models in the public domain. Recent relevant examples do, however, include the Urban Transport Roadmaps scenario tool (European Commission, 2016). The tool allows the development and assessment of a number of sustainable transport scenarios. Including the introduction of modal shift measures - bike sharing, walk and cycle infrastructure, bus and tram network and the prioritisation of public transport and demand management measures - congestion charging and parking charges. Output of relevance includes associated reductions in air quality pollutants and road accidents. The Defra model - Assessing the Wider Impacts of Air Quality Policies captures outcomes associated with changes in vehicle kilometres including (Green et al., 2015):

  • Congestion
  • Safety
  • Noise

While the model references Air Quality Policies the focus on outcomes associated with changes in vehicle kilometres means that it is applicable in this carbon mitigation context. The user can select where changes take place i.e. Scotland can be selected. The model follows the same principles outlined above in that inputs relate to changes in vehicle km driven. The model draws heavily from the DfT's Marginal Economic Costs including its approach for monetisation of noise. Thus consideration needs to be given to how this would fit with STAG's current approach.


Email: Debbie Sagar

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