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4 The Assessment Process

Transport or land-use projects have the potential to affect air quality if there are changes in traffic flows, speed, congestion or traffic composition. If these changes are significant, an air quality assessment may be required. This section provides guidance on assessing the air quality effects at a level of detail appropriate to the likely effects of the project.

Note that projects involving maintenance works and/or minor safety upgrades are not likely to have significant air quality impacts and do not require assessment.

4.1 Key steps in the assessment process

An assessment of discharges to air from land transport will typically involve the following steps.

  1. Gather information and make a qualitative preliminary assessment of air quality impacts.
  2. Liaise with relevant local authorities.
  3. Predict the contribution of the proposal to ground-level concentrations.
  4. Characterise the existing environment.
  5. Assess the cumulative air quality impacts.

In practice the process may be simpler - or more complex. For example, a proposal with minor impact would generally only require a preliminary assessment.

Each of these steps should be undertaken to a level of detail that is appropriate to the nature and scale of the proposal. This guide suggests a three-tiered approach to assessment, as discussed below. A brief description of each component of the process is provided in this section, with more detail given in the following sections of the guide.

Make a preliminary assessment

The first step in any assessment of air quality impacts is to compile information and make a qualitative assessment of the likely impacts. For some proposals this preliminary assessment may be all that is needed. However, in most cases the purpose of this stage is to identify key issues early in the process.

The preliminary assessment is referred to as a Tier 1 assessment in this guideline.

Liaise with relevant local authorities

For any significant assessment of discharges to air from land transport, the relevant council should be involved early in the process.

Predict the contribution of the proposal to ground-level concentration of air pollutants

Predicting the likely contribution of the proposal to ground-level pollutant concentrations requires an estimate of emissions and future trends. Traffic data are combined with emission factors (from an emissions model or database), and emissions are calculated accordingly.

Atmospheric dispersion modelling is often used to predict the likely contribution of the proposal to ground-level pollutant concentrations. Detailed guidance is provided in the Ministry for the Environment’s (2004) Good Practice Guide for Atmospheric Dispersion Modelling. Transport projects requiring dispersion modelling would, in most cases, require the use of models that are designed specifically for transport emissions assessments (eg, CALINE4).

Characterise the receiving environment

The potential impacts of the proposal on air quality must be considered in light of the existing air quality, the sensitivity of the receiving environment, and the local topography and meteorology.

Assess the cumulative impacts of the proposed discharges

Transport projects can cause a range of environmental effects, including human health effects and effects on ecosystems. These effects can generally be assessed by comparing the predicted ground-level concentration of pollutants (including existing background concentrations) with appropriate assessment criteria. Air quality assessment criteria are discussed in section 5.

4.2 The level of assessment required

Section 88 of the RMA requires an assessment of environmental effects to be provided “in such detail as corresponds with the scale and significance of the effects that the activity may have on the environment”. This section provides guidance on the level of assessment that is appropriate for land transport assessments. Figure 4.1 illustrates the overall assessment process recommended in this guideline. The three-tiered approach is intended to ensure that the level of assessment undertaken reflects the likely level of effect from a proposal.

The three tiers of assessment are:

  • Tier 1: preliminary assessment, to identify whether there are likely to be significant air quality effects

  • Tier 2: screening assessment, using straightforward dispersion modelling techniques

  • Tier 3: full assessment, with increased complexity in modelling and reliance on site-specific data.

Tier 1 and Tier 2 assessment procedures apply only to the assessment of emissions from motor vehicles. Assessment of other land transport projects (eg, railways and tunnels) requires specialist input. The focus of the Tier 3 assessment is also on emissions from motor vehicles, but the same general principles are applicable to any transport project or traffic-generating development.

Figure 4.1: Components of the air quality assessment process

4.2.1 Tier 1 assessment

Any transport or land-use project has the potential to affect air quality and exposure risk if there are changes in traffic flows, speed, vehicle fleet composition, congestion, or the location of traffic relative to receptors. The Tier 1 preliminary assessment is a qualitative assessment to determine whether there are likely to be adverse effects. The level of detail required will vary depending on the nature of the proposal. Some projects that are unlikely to result in significant increases in emissions will not require further assessment, and the Tier 1 assessment suggests quantitative criteria to identify these projects. These criteria are intended as a guide only. The extent of the air quality assessment is a matter for judgement.

For larger projects the Tier 1 preliminary assessment provides an opportunity to identify key air quality issues early in the process.

4.2.2 Tier 2 assessment

Tier 2 is a relatively quick and easy quantitative assessment. A Tier 2 screening dispersion-modelling study provides conservative estimates of likely air quality impacts. This means the assessment can provide confidence that a project will not result in significant air quality impacts, despite the relative uncertainty of the predictions. If this screening assessment indicates there is a potential for adverse impacts or non-compliance with air quality criteria, then the modelling and assessment approach may need to move up to the Tier 3 assessment level, with the modelling further refined to increase the accuracy of the estimates, enabling some of the conservativeness of the assessment to be removed.

A Tier 2 assessment should be undertaken for any project (or part of a project) that is identified from the Tier 1 assessment as needing further assessment, to determine whether significant adverse effects on air quality are likely. For major projects it may be immediately obvious that a Tier 3 assessment will be required. However, Tier 2 may still be useful for doing a quantitative ranking of options and for identifying parts of the project that require detailed assessment.

4.2.3 Tier 3 assessment

A more detailed Tier 3 assessment is required if the screening assessment indicates potential significant adverse effects, or if the project is sufficiently complex that a screening assessment is not appropriate. As stated earlier, for some projects the need for a Tier 3 assessment may be obvious during the preliminary assessment.

It is recommended that the relevant council with responsibility for air quality management be consulted before undertaking a Tier 3 assessment.

A Tier 3 assessment may require site-specific emission calculations, including ambient air monitoring data (to confirm background concentrations), and atmospheric dispersion modelling. In many cases the tools or models are the same as those used for a Tier 2 screening assessment but require more detailed input data. For example, an atmospheric dispersion model may accommodate real-time meteorological data, or at least a representative meteorological frequency distribution, rather than simple worst-case assessments.

Detailed assessments may also be required to handle special localised features, which may affect the dispersion of contaminants that are not adequately examined by the screening models. This could include areas of complex terrain, severe street canyons, complex intersections and discharges from road- or rail-tunnel vents.

4.2.4 Deciding which tier

The concept of splitting the assessment methodology into three tiers is somewhat arbitrary. The aim is to provide some guidance – particularly to people new to the field – on the different levels of work and detail needed. For instance, looking crudely at the resources required:

  • Tier 1 probably requires just several hours of work, using existing documentation and information.

  • Tier 2 probably requires several days of work, including the generation of some new results, and probably a report.

  • Tier 3 probably requires several weeks (or months) of work, including advanced modelling, possibly new monitoring, the use of a number of experts, and probably several reports, with peer review (such as for a new large road, near sensitive receptors, in a poor quality airshed). These types of activities are those most likely to be appealed to the Environment Court.

The delineation between the tiers is not absolute. Basically, a Tier 2 or 3 assessment should be undertaken to a level of detail sufficient to determine the air quality effects in a robust and defensible manner.

4.3 Factors affecting the level of assessment

The factors affecting the level of assessment required include the:

  • scale of the development

  • extent of any increase in emissions

  • scale of effect

  • whether the project is within an airshed that is likely to exceed the national environmental standards for air quality (the Standards)

  • existing air quality

  • physical geography of the receiving environment

  • land use of the receiving environment

  • any legislative or consent requirements.

The Tier 1 preliminary assessment process includes a qualitative assessment of these factors. In some cases it will be obvious from the preliminary assessment and discussions with the regional council that a Tier 3 assessment will be required. However, a Tier 2 screening assessment may still be undertaken to identify the pollutants or sources of most concern before undertaking a Tier 3 assessment.

In reality, it is likely that the methodologies described in the Tier 2 and Tier 3 procedures would be combined for any significant assessment. For example, a scarcity of information on existing air quality may result in the need for air quality monitoring, whereas the required dispersion modelling technique may be very straightforward due to simple topography.

One of the advantages of early liaison with the relevant local authority is the opportunity to get advice on what level of quantitative assessment is likely to be required based on their experience with similar proposals. In this way, the assessment process can be targeted at the appropriate level rather than wasting time by following an iterative approach of increasing detail and complexity.

For major projects, the level of assessment required will also depend on the stage or status of the project. A major project will have several assessment stages before consideration of the notice of requirement, including feasibility studies, scheme assessments and detailed design. For these types of project it is important that a preliminary Tier 1 assessment be undertaken early in the process so that air quality issues can be identified and considered in the project design. For example, if there are three choices of route at the scheme assessment phase, but one option takes the route near a rest home or school, then from an air quality perspective the other two route choices may be preferable.

4.3.1 Legislative and policy requirements

Legislative and policy requirements are outlined in section 2. It is important to consider any requirements of relevant legislation or policies in determining the appropriate level of assessment.

4.3.2 National environmental standards for air quality

At the time of publication, 69 areas throughout New Zealand have been gazetted as 'airsheds' under the provisions of the Standards. Of these, around 30 are likely to exceed the national ambient air quality standard for PM10 and are therefore subject to controls on the granting of resource consents. It is likely that any development within these airsheds that shows any significant increase in PM10 emissions will require a Tier 3 assessment.

The additional requirements imposed by the Standards (as well as those already in council plans) cover many other pollutants aside from PM10. For instance, some regions have an issue with NO2 (mainly in Auckland, due to its high level of traffic emissions) and some with SO2 (mainly due to industries using coal).

4.4 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 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. However, 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 an exceedance of the national 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 Appendix 4.

An assessment of the impacts of a transport project cannot be carried out in isolation, but 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.3

In major developments it is important to consider environmental impacts early in the process. For example, route choices should not be considered and eliminated before considering air quality impacts. 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. 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 exceedances of the national ambient air quality standards, so it is advisable to undertake a quantitative assessment (Tier 2 or Tier 3) of local air quality impacts as soon as practicable in the project evaluation process.

4.5 Reporting

It is a good idea to develop a standard reporting methodology and format. Although specific projects will have specific requirements, any report should contain the following features.

  1. Executive summary: a one-page statement of the key features and results. This may be the only part of the report that some users read, so it should be succinct and clear.
  2. Scope: who has commissioned the project, and why, including the intended outcomes.
  3. Introduction: the background to the issues and the relevance of any previous work.
  4. Methodology: a description of the process used, any models employed, assumptions made, and any statistics or analysis used.
  5. Site description: the area being assessed, including maps with all relevant features (and photos if available). Show any facilities where there are likely to be sensitive members of the public (eg, young, old, sick), such as hospitals and schools, in relation to the proposal.
  6. Description of proposal: a description of the project and any changes to existing activities. This section should include enough information to characterise the discharges and the receiving environment, as detailed in later sections of this guidance. This section should also include a clear description of the options being considered.
  7. Description of the receiving environment: a description of the receiving environment as detailed in later sections of this guide.
  8. Data used: the sources and validity of all input data, including traffic flow data, fleet profiles, emission factors, meteorology, existing concentrations and all assumptions made.
  9. Assessment of effects: the outcomes of the study, and all options assessed, as much as possible in summary tabular and graphic form. The emphasis should be on key results that can inform decision-making. Where possible, errors should be estimated and stated. Detailed results should be given in an appendix.
  10. Discussion: any implications, uncertainties and reliance on assumptions. Include discussion of possible mitigation options and associated emissions reduction.
  11. Conclusion: a summary of the scope, method, result and implications.
  12. References: all material used should be referenced explicitly, and should include web-based links where appropriate.
  13. Appendices: any detailed calculations or results that are used. This should include model control files.

The size and nature of each of these sections will depend on the project, but it is anticipated that for any Tier 3 assessments each section will be included and the report will typically run to 30 to 60 pages. Specific requirements for Tier 1, 2 and 3 assessments are discussed in sections 6, 7 and 8 of this guide.

4.6 Reasonableness

Assessing air quality effects is a complex task. Although a number of quantitative tools are applied, many of these have uncertainties that at times can be difficult to quantify. Many steps in the process require assumptions to be made, and this is where the concept of 'reasonableness' comes in.

For instance, although in theory it would be possible to describe and model all possible options (all types of traffic, all types of drivers, all weather conditions, etc), this would result in hundreds or even thousands of sets of results, for an unreasonable cost. It is also remarkably difficult to gain accurate information on future emissions characteristics, particularly with evolving engine technologies and the sometimes unrealistic performance of transport models out beyond a few years.

Even if very accurate emissions and dispersion modelling for the project under consideration are possible, the situation is further compounded by the fact that the state of knowledge about other sources and their effects is imperfect, particularly in areas that are geographically complex and have many other sources and no ambient monitoring (ie, some New Zealand cities).

Calls are often made - particularly by the various engineering professions responsible for the design, implementation and operation of the transport networks - to provide quantified uncertainty estimates. However, the current state of knowledge about emissions and dispersion is such that this is simply impossible to do with the same rigour as most engineering calculations. Simple uncertainty estimates at each stage can compound to create enormous uncertainties in the final results. This is not so much looseness in the computations as a feature of not having developed the right statistical methods (for instance, many processes are far from being normally distributed and/or are often not independent, so common statistical measures of uncertainty are inappropriate).

This lack of rigorously quantified uncertainty does not detract from the value of the methodology used. Indeed, this is precisely why so much emphasis is placed on monitoring, validations and experience from similar projects.

As a result, any assessment of the air quality effects of transport needs to apply a factor of reasonableness. This is difficult to define, but should include consideration of the following factors.

  • Fleet composition: Is it accurate enough? Does it represent the conditions of interest?

  • Transport corridor being assessed: How far does it go? Are the right number of feeder routes included?

  • Project options: Are there a manageable number (not just one or two, but also not 50 or 60)?

  • Time horizon: How far into the future is reasonable? One year is not enough, but 30 is probably too much.

  • Contaminants: Are the right ones on the list? Are there not too many, but not too few?

  • Existing air quality and effects: Are the time and spatial variability captured?

  • Cumulative effects: Is the procedure for examining the additive effects of the project emissions to existing effects robust?

  • Receiving environment: How far to go − right on the road, or 10 m, or 200 m?

  • Reporting: Just enough to tell the story? Or should there be hundreds of pages of background and justification?

  • Mitigation options to assess: Just what might be sensible and practical versus unrealistic or cost prohibitive?

The rest of this good practice guide should help with deciding these questions, but the answers to them will never be absolute.

The assessment process - recommendations

An assessment should be undertaken in such detail as corresponds with the scale and likely significance of the effects. A three-tiered approach is outlined as a guide to the different types of information required and the different levels of resources required to complete the application.

It is important that air quality is considered early in a transport project. A Tier 1 preliminary assessment should be undertaken early enough so that air quality issues can be identified and considered in the project design.

Air quality should be considered when comparing project options (including alternative routes). This comparison should include a qualitative (Tier 1 preliminary assessment) and, where possible, a quantitative (Tier 2 or 3) assessment of the air quality impacts of different options. A quantitative method for estimating overall health impacts and associated costs is further recommended in Appendix 4.

A level of 'reasonableness' needs to be applied in any assessment of the air quality effects of transport (based on the judgement of experienced practitioners).

A reporting format is suggested for any assessment of environmental effects reports to ensure consistency and avoid omissions of important relevant material.


3 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.