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Good Practice Guide on Assessing Discharges to Air from Land Transport

Foreword by the Ministry

Acknowledgements

Key Data Sources & Models

1 Introduction

2 Legislative and Policy Context

3 Assessing Discharges to Air from Transport

4 The Assessment Process

5 Air Quality Criteria

6 Tier 1 Screening Assessment Process

7 Tier 2 Conservative Assessment Process

8 Tier 3 Detailed Assessment Process

9 Assessment of Community Impacts and Comparison of Project Options

10 Mitigation Options

11 Monitoring Project Effects

References

Appendix 1: Calculations and Data

Appendix 2: Traffic and Transport Data

Appendix 3: Vehicle Emissions and Effects

9 Assessment of Community Impacts and Comparison of Project Options

9.1 Displacement

An assessment of the impacts of a transport project cannot be carried out in isolation, and it is vital to take account of the wider aspects. For instance, a mitigation measure on one particular road segment, designed to improve air quality around that road, may have consequences for increased emissions elsewhere. This can occur through increasing congestion, re-routing traffic through a more sensitive area, increasing total volumes in the area, or altering the vehicle mix.

There are numerous examples throughout the world of this, such as in the Swedish city of Stockholm. Expensive measures to heavily control the CBD traffic did not decrease the total amount of excessive air pollution − it simply moved it out of the CBD and into a wide band around the city where the new ring roads had been constructed. [For a discussion of the Stockholm displacement problems, and similar issues in other cities, see Sustainable Mobility Assessment & Renewal Technology for Capital Improvements of Transportation Infrastructure (SMARTCITI), Proceedings of the 9th Intercontinental Conference, Seattle, Washington, 29 June − 1 July 2005.]

9.1.1 Assessing community impacts

Most transport projects are designed to improve safety and traffic flows, and will therefore have a net air benefit for the community. For significant projects that increase emissions in some locations and decrease emissions in others, it may be useful to quantitatively assess the overall community impact. Methods for quantifying community impacts are recommended in section 9.3 below. The method selected will depend on the level of detail required.

This guide recommends methods for assessing community impacts, but it is not possible to provide guidance on the relative importance of community versus local impacts. For example, construction of a new link may ease congestion and reduce overall vehicle kilometres over a wide area, but air quality immediately adjacent to the new link may be degraded. Although the net effect of the project would be to maintain or improve overall air quality, an assessment of local impacts is still required to determine whether any localised negative impacts are within acceptable limits, and whether mitigation can or should be applied.

In cases where air quality is significantly degraded across the community, an assessment of air quality impacts across the relevant airsheds affecting that community may be required. Airshed assessments are discussed in section 8.4.3. Depending on the state of air quality in the airshed, the project might require offset/mitigation. It is recommended that the regional council be consulted to determine assessment requirements for any proposal that will result in a significant net increase in emissions.

9.2 Comparing project options

Air quality is one of many issues that need to be considered in comparing project options. Transport projects are evaluated against the five objectives of the New Zealand Transport Strategy, which are:

• assisting economic development
• assisting safety and personal security
• improving access and mobility
• protecting and promoting public health
• ensuring environmental sustainability.

Projects can also be evaluated against other central, regional or local government objectives and policies. The legislative and policy framework is discussed further in section 2.

For major projects, there are often compromises between objectives. For example, an option that improves safety by providing pedestrian facilities, may increase vehicle emissions and degrade air quality. In these cases, it can be difficult to assess the relative importance of air quality compared with other objectives. There are two key questions that can help in comparing project options:

1. Is the option likely to result in any significant localised adverse effects? In particular, is the option expected to result in a breach of the ambient air quality standards, and can this be mitigated?
2. What is the estimated overall health cost (value) from air pollution for each option?

A quantitative method for estimating overall health impacts and associated costs is recommended in the following section.

At the early stages of project evaluation, quantitative comparison may not be possible. At this stage, it is recommended that comparison of projects be based on a qualitative assessment, as described in the tier 1 assessment section.

9.3 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 LTNZ methodology.

These approaches are discussed in more detail below.

9.3.1 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. Calculate total emissions (tonnes per year) of PM₁₀ for each study area.
2. Estimate the population for the study area.
3. Determine the area under study (km2).
4. For each study area, calculate emissions per km2 x 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.

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.

9.3.2 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.

Equation 9-1

Effect = health effect factor x ΔPM₁₀ x population exposed x normal death rate x value of life

where:

• 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 percent 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 www.stats.govt.nz)
• value of life is derived from the analysis conducted by the Land Transport Safety Authority (LTSA) 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 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 µg m^-3 in some work, and also nominally accounts for natural background levels (however, in the cases being examined here this would not generally apply, since in most urban areas the concentration is already above 7.5 µg m-3 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 upon used this.

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 ΔPM₁₀ 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 (ΔPM₁₀ 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 9-1: Banding and weighting of property counts

* 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 9-2

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

Σ_{each link} {ΔPM₁₀ roadside x number of weighted properties on link x property occupancy rate}

9.4 Local impacts

An assessment of local impacts is required regardless of the overall impacts to ensure that any local adverse effects are within acceptable limits. The method for assessing local impacts has been discussed in detail in previous sections. However, at the early stages of project evaluation, there is often inadequate information − or simply too many options − to assess local impacts. In this case, a qualitative assessment (Tier 1) can help with project ranking. This should consider the likely air quality issues for each project option, such as:

• the background air quality
• any areas where dispersion is likely to be poor (valleys or building canyons)
• the location of any traffic "hotspots" (intersections, congested areas, busy areas) in relation to any areas with poor dispersion or sensitive receptors
• the location of any sensitive receptors (residential areas, hospitals, schools, etc) in relation to the development, and in relation to any likely hotspots or areas with poor dispersion
• the existence of any location-specific plans, policies or community requirements.

Mitigation will need to be considered for any option that results in breaches of the ambient air quality standards, so it is advisable to undertake a quantitative assessment of local air quality impacts as soon as practicable in the project evaluation process.