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Appendix 4: Valuation and Comparison of Community Air Pollution Effects

Two approaches are recommended, depending on the level of detail available. These are based on the UK Department for Transport’s (2004) Transport Analysis Guidance and the Design Manual for Roads and Bridges (National Roads Authority and DEFRA, 1992/2003).

  • At a strategic or spatially coarse level (where individual link traffic flows and speeds may not be available), quantify the change in total emissions and relate it to the local population density. This method provides a spatially coarse exposure index, and does not provide for the valuation of effects.

  • At a project level, quantify the change in exposure at nearby properties. This provides the data necessary to estimate the 'value' of the effects, based on Land Transport New Zealand methodology.

These approaches are discussed in more detail below.

A4.1 Strategic level – health effects index

It is recommended that a strategic-level assessment be used for preliminary comparison of project options if there is not enough information to undertake a project-level assessment. The recommended approach for option assessment at a strategic level will provide a relatively crude index of likely health effects, which can be used to compare options, as follows.

  1. Select study areas.
  2. Calculate total emissions (tonnes per year) of PM10 for each study area.
  3. Estimate the population for the study area.
  4. Determine the area under study (km2).
  5. For each study area, calculate emissions per km2x population density.

This may provide better information than simply comparing total emissions, because there is some consideration of population density. However, this will depend on the variability of population density and the size of the study areas selected. If the 'study area' is unchanged in each scenario, then the population density will not be relevant. However, if the study areas are carefully selected, the approach will allow two options that may yield the same total benefits, in terms of the change in tonnes of emissions, to be differentiated if one tends to favour emissions savings in populated areas.

This method is also useful when making any comparison with standard emissions inventories, which are available for many regions. These can generally be obtained directly from the relevant regional council or through their website.

This method should only be relied on for preliminary studies (Tier 1). The health effect index provides an indication of likely relative effects, but relies on the assumption that a reduction in emissions will lead to a reduction in the population exposure across the study area. This may not always be the case; for example, options that have a bigger separation distance between the road and adjacent residences may result in lower overall exposure than options with lower emissions and smaller separation distances.

A4.2 Project level – valuation of community air pollution effects

At the project level, the aim is to calculate the likely health effects as a result of change in exposure at properties for each option. This needs to take account of all changes in exposure, whether on existing or new routes. The recommended approach provides an indication of the likely cost associated with overall health impacts. This can assist in comparing project options.

To compare options, it is recommended the likely health costs associated with the 'do minimum' option and each other option are calculated using the following equation from section A8.2.5 of the Project Evaluation Manual (Transfund, 2004). This is a simplified approach, but is accepted as a standard method in many countries, and it is the approach used by the World Health Organisation when making international assessments. The Transfund methodology relies on PM10 to estimate health effects. This is a valid approach in most cases because the effects of PM10 tend to dominate. However, the same approach could be applied to other pollutants (for example, NO2 in Auckland).

Equation A4- 1

Effect = health effect factor x ∆PM10 x population exposed x normal death rate x value of life


  • health effect factor is the percentage increase in daily mortality for a 1 µg/m3 increase in PM10 concentration (it is currently recommended that a value of 0.43% be used; Fisher et al, 2005)

  • ΔPM10 is the change in annual average PM10 concentration (µg/m3)

  • normal death rate is taken from the published life tables in New Zealand and is currently 7.53 per year per thousand people, calculated in 2003 (normal death rate data are available from

  • value of life is derived from the analysis conducted by the NZ Transport Agency (NZTA) and Ministry of Transport in relation to crash deaths. It is not the same as a crash death, because air pollution affects people differently to crashes (see Fisher et al, 2005). It is recommended that a value of $750,000 be used.

The above relationship is simplified and is given here to demonstrate the process. Some evaluations use a threshold value, assuming that for low values of annual PM10 the effect is negligible. This has been set at 7.5 g/m3 in some work, and also nominally accounts for natural background levels (although, in the cases being examined here this would not generally apply, since in most urban areas the concentration is already above 7.5 g/m3 and the incremental effect needs to be quantified. The relationship should also be used only on the population over 30 years old, since the work it is based on used this population.

The average change in PM10 concentration across the population exposed can be determined through dispersion modelling studies. Alternatively, the following methodology adapted from the Design Manual for Roads and Bridges (National Roads Authority and DEFRA, 1992/2003) can be used to calculate ∆PM10 concentration x population exposed.

  1. Estimate the roadside annual average PM10 concentration for the 'do minimum' and for each option, for each road or link being considered. Annual average PM10 concentrations can be calculated based on the method described in Appendix 1.
  2. For each affected road (new road and existing), calculate the difference in roadside levels of PM10 (∆PM10 roadside) between the 'do minimum' and proposed scenarios. A positive value should be assigned where an increase in concentration has been identified, and a negative value for a decrease in concentration.
  3. For each affected road, estimate 'banded' property counts. The properties should be banded and the number of properties within each band recorded. These property counts should then be weighted using the factors below. This weighting accounts for the reduction in pollutant concentration with distance from the road. It is assumed that beyond 200 m the contribution of vehicle emissions from the roadside to the local ambient air concentrations is not significant.

Table A4.1: Banding and weighting of property counts

Bands (measured from edge of the carriageway)


Roadside to 50 m from roadside


51 to 100 m from roadside


101 to 150 m from roadside


151 to 200 m from roadside


* Weightings are based on the annual average dispersion curve developed by Scoggins et al, 2004.

4. The overall health cost for each option can then be estimated based on the following equation:

Equation A4- 2

Effect = health effect factor x normal death rate x value of life x

∑each link {∆PM10 roadside x number of weighted properties on link x property occupancy rate}