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7 Tier 2 Screening Assessment

The Tier 2 assessment is a relatively simple screening exercise to determine whether a proposal is likely to result in exceedances of ambient air quality criteria, in particular the national ambient air quality standards.

The aim of a screening assessment is to provide conservative estimates of air quality effects. These may not be completely accurate, but they can provide confidence that a project will not result in significant air quality effects. The recommended screening assessment process is based on the methods most commonly used in New Zealand.

The Tier 2 and Tier 3 assessments use essentially the same tools and techniques. However, this section suggests conservative default values for traffic data, emission factors and dispersion modelling, so that a Tier 2 assessment can be undertaken relatively quickly and easily. Users who are not familiar with the assessment process should read this section in conjunction with section 8 (Tier 3 assessments).

In general, the Tier 2 assessment should be undertaken for any transport links and intersections identified in the Tier 1 assessment. For projects that affect multiple links or intersections, the worst cases can be assessed to determine whether problems are likely at other locations. The worst case will be the location with the closest receptors and highest emissions.

For changes to existing roads or networks, the assessment must be undertaken both with and without the proposal so that the impacts of the proposal can be clearly established.

7.1 National environmental standards for air quality

Special care may be needed for any assessment in an airshed that exceeds the national ambient air quality standards. Consultation with the regional council is recommended before finalising any detailed Tier 2 assessment within an airshed that already exceeds the national ambient air quality standards.4

Figure 7.1: Tier 2 assessment of local impacts

7.2 Characterising the discharges to air

Characterising the discharges to air for a Tier 2 assessment includes:

  • estimating the effect of the proposal on traffic

  • using emission factors to estimate the change in emissions as a result of the proposal.

The following sections provide guidance on estimating traffic effects and emissions for a Tier 2 air quality assessment.

7.2.1 Traffic data requirements

Traffic flow

The assessment ideally requires an estimate of 24-hour, eight-hour and one-hour traffic flows for a high-traffic day. Currently, annual average daily traffic (AADT) counts are often the only available traffic data. AADT is an estimate of the average traffic count over a year, and on any given day traffic may be considerably higher or lower than the AADT. Newer traffic models being employed are capable of giving more detailed and accurate information, down to an hourly or finer resolution, for each link at various times of the day, week or year. Whenever possible, the best data should be used, preferably at least at hourly resolution.

In some cases, peak and inter-peak hourly data are available, but these are generally the peak and inter-peak hourly traffic for the annual average day. The aim of a screening assessment is to predict worst-case air quality. Therefore, the traffic analyst should provide an estimate of daily one-hour and hour-hour traffic flows on a high-traffic day based on local traffic count information, or on traffic count information from a similar location. Traffic count data are generally available from territorial local authorities (or Transit for state highways).

It is essential that the air quality assessor understands the basis, meaning and limitations of any traffic data.

For an assessment where only daily traffic flow data are available, the one-hour peak and worst eight hours should be estimated from traffic count data.

Auckland traffic count data, which were compiled for the Auckland Regional Council emissions inventory, showed that the highest proportion of vehicle kilometres travelled (VKT) over one hour was approximately 7–8% of the 24-hour traffic count, occurring from 5 pm to 6 pm. The highest proportion of VKT in eight hours was approximately 54% of the 24-hour traffic count, occurring between 11 am and 7 pm. Therefore, for the purposes of assessment, one-hour and eight-hour traffic could be estimated as 10% and 60% of the 24-hour traffic flow, respectively.

Average speed

Vehicle emission factors are sensitive to speed and level of service (congestion level). In most cases, a Tier 2 screening assessment can be based on worst-case emission factors, so speed data are not required. Where speed data are readily available (eg, the local council may have SCATs data at intersections), they can be used to help select the most appropriate emission factors. However, the assessor should justify that the emission factors are conservative. Selection of emission factors is discussed in section 7.2.2 below.

Vehicle fleet composition

Vehicle fleet composition is an important parameter in air quality assessments because of the relative health importance of different pollutants. The health effects of air pollution are dominated by particulates, and emissions of particulates from transport are dominated by heavy commercial vehicles (HCVs).

For a Tier 2 screening assessment, vehicle fleet composition data are typically not required unless a high proportion of HCVs is expected compared to the default of 7.6% HCVs over 3.5 tonnes (Auckland Regional Council, 2005a). This might occur, for example, on busways, port access roads and quarry access roads. Default emission factors are provided in Table 7.1 for the national average fleet composition (as provided in Table 8.1). For assessments where there is a higher proportion of HCVs (compared to the default), emission factors will need to be derived from the New Zealand Traffic Emissions Rate database (NZTER), as described in section 8.1.3.

Table 7.1 Fleet-weighted exhaust emissions factors for 2004 (NZTER)

Road type

Congestion level

Fleet-weighted exhaust emission factors (g/km)







Central urban






















Cold start





























Cold start





























Cold start







Fleet-weighted average







* NZTER provides total suspended particulate (TSP) exhaust emission factors. It can be assumed that 100% of exhaust TSP is less than PM2.5.

7.2.2 Emission factors

The Ministry of Transport’s Vehicle Fleet Emissions Model and associated NZTER are currently the preferred sources of emissions information for the New Zealand fleet. The limitations and applicability of NZTER are discussed in section 8.1.3. Updated emission factors are being developed, but until these become available NZTER is considered appropriate for Tier 2 screening assessments.

For a Tier 2 screening assessment, the most conservative approach for calculating one-hour emissions from a transport corridor would be to assume the worst-case level of service (LOS) for the type of road being assessed. LOS is a representation of the level of congestion on a road. The appropriate (highest) emission factors are for congested conditions, except for NOx and PM10 emission factors for motorways. Higher speed limits on motorways mean that NOx and PM10 emissions are higher under free-flow conditions.

A conservative prediction of eight-hour and 24-hour emissions in an urban area therefore assumes congested emission factors for peak hours (eg, four hours of the eight-hour and 24-hour averaging periods) and interrupted emission factors for the remainder of the eight-hour or 24-hour period. Ultimately, the assessor will need to use his or her judgement, and justify that the emission factors selected are appropriately conservative for a screening assessment, recognising that the assessment should consider worst-case locations. So, for example, if the air quality near an intersection is being assessed, the emission factor should be selected for the expected LOS in that location, as opposed to the average speed or LOS for the link. In all cases, the assumptions used and their justification should be clearly reported.

Cold-start emission factors represent emissions from vehicles that have not warmed up. This effect is most significant in catalyst-equipped petrol vehicles. The emission factors are high because emissions control equipment does not function well until the vehicle is warm, and until then the combustion process is not as efficient as it can be. Cold-start emission factors should generally apply for the first three minutes of vehicle travel (or at least 2.5 km). For the purposes of a screening assessment it should be assumed that 20% of all vehicles are operating under cold start conditions in urban areas (Auckland Regional Council, 2005a). In practice, cold-start rates may be lower depending on the route and the feeder roads; for instance, the Auckland-wide average rate, from the regional traffic model, is 17%. In a study comparing modelled emission rates with those measured using an on-road remote sensing device, better agreement was obtained when assuming cold-start rates of 10% or greater (Bluett and Fisher, 2005). In the absence of specific data on this factor from a reliable traffic model, the 20% figure is an appropriately conservative choice.

Use of speed-dependent emission factors

It is likely that NZTER emission factors will be updated or replaced with speed-dependent emission factors for the New Zealand fleet in the near future. For a screening assessment using speed-dependent emission factors, it will be necessary to select an appropriate speed. As we have seen, vehicles generally emit lower amounts of pollutants when they are travelling at their optimal speed (30 to 70 km/hour for most vehicles). Emission rates tend to increase at higher and lower speeds.

Speed data may be available from the results of traffic modelling or monitoring. However, the assessor needs to demonstrate that the average speed selected is conservative for the location and averaging period being considered. Where speed data are not available, a worst-case assumption can be made based on the typical speed ranges given in Table 7.2. When there is doubt about the likely speed, the lowest speed within the range should be used.

Table 7.2: Typical speed ranges for use with speed-dependent emission factors

Traffic condition/location


Around intersections

10 km/hr

Urban or suburban areas that experience congested conditions during rush hour

10–25 km/hr for a 1-hour average

Urban or suburban roads with interrupted flow (some traffic lights, etc)

25–45 km/hr

Free-flowing urban or suburban roads

45 km/hr

Free-flowing motorway

80 km/hr

7.2.3 Traffic data and emission factors for additional future years

Predicting the likely future impacts of transport projects is difficult. Any prediction requires an estimate of likely future traffic, as well as the fleet composition and emission factors. It is expected that fleet-weighted emission factors will reduce over time as more modern, lower-emission vehicles enter the fleet and older vehicles retire. NZTER includes emissions factors for future years, based on predicted changes in the fleet and emissions technology. However, there is a great deal of uncertainty about these predicted emissions.

In general, it is typically assumed that the rate of reduction in fleet-weighted emission factors will be higher than the overall rate of growth in traffic (except for CO2). A Tier 2 assessment for additional future years should only be necessary if it is expected that the rate of traffic growth (after the assessment year) on the road or link under consideration will be significantly greater than expected overall traffic growth rates (say 2–3% per annum), or if it is expected that the LOS will significantly decrease, or if the proportion of heavy-duty vehicles will significantly increase for some reason.

For future years, a Tier 2 assessment using the latest 2004 emission factors will provide a conservative assessment of likely impacts. If this assessment indicates that significant impacts are likely, then emission factors from NZTER for the year being assessed could be adopted. However, this should be done with caution.

The analyst should document predicted traffic for future years (with and without the project, where appropriate) and justify whether to assess likely future impacts. The future assessment years may depend on the availability of traffic demand models, but should include a long-term forecast (at least 10 to 20 years).

Where future emission factors are adopted, the limitations of these need to be recognised and discussed. This also applies to any projects currently being considered which will not be constructed immediately.

7.3 Exposure estimates

Appendix 1 shows a screening method for estimating the traffic-derived, ground-level concentrations of carbon monoxide (CO), nitrogen dioxide (NO2) and fine particulate (PM10). This method uses simple techniques and relatively crude assumptions, with the aim of ensuring conservative estimates. Alternatively, a near-road dispersion model, such as CALINE4, may be used with worst-case meteorology to estimate the traffic-derived, ground-level concentrations of air pollutants. The use of CALINE4, as well as other corridor dispersion models, is discussed in the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004).

7.4 Assessing the effects

7.4.1 Tier 2 assessment of effects

To assess whether significant air quality impacts are likely, the maximum ground-level concentration predicted by the screening methodology should be compared to the significance criteria in Table 7.3. This comparison should be made for locations where people may be exposed for the relevant averaging period. A Tier 3 assessment should be undertaken if the predicted concentration of PM10, NO2 or CO due to the proposal exceeds any of the assessment criteria. A Tier 3 assessment should also be undertaken if the Tier 2 assessment is inconclusive for any reason.

The Tier 2 assessment is intended to provide a conservative estimate of the likely air quality impacts of a project. If these criteria are exceeded, this does not necessarily mean the air quality impacts will be significant; it simply means a more accurate assessment should be undertaken. Mitigation options, or alternative options that do not exceed the criteria, could be considered at this stage, but it should be noted that further assessment may show these are not required.

Table 7.3: Significance criteria for incremental analysis only



Significance criterion

Time average


50 µg/m3

2.5 µg/m3

24 hours


25 µg/m3

1.3 µg/m3

24 hours

Nitrogen dioxide (NO2)

200 µg/m3

20 µg/m3

1 hour

100 µg/m3

5 µg/m3

24 hours

Carbon monoxide (CO)

10 mg/m3

1 mg/m3

8 hours

In general, the Tier 2 assessment should be undertaken for all links and intersections identified in the Tier 1 assessment. For projects that affect multiple traffic links or intersections, the worst cases can be assessed to determine whether problems are likely at other locations. The worst case will be the location with the closest receptors and highest emissions.

Other pollutants do not need to be assessed for a Tier 2 screening assessment. CO, NO2 and PM10 provide a good indicator of transport effects and are the pollutants of most concern. If the assessment shows that levels of these contaminants are within acceptable criteria, then the analyst can be reasonably confident that levels of other traffic-related pollutants will be acceptable.

The primary reference criteria are the national ambient air quality standards provided by the Resource Management (National Environmental Standards Relating to Certain Air Pollutants, Dioxins and Other Toxics) Regulations 2004. However, for Tier 2 assessments, compliance with the Standards is not explicitly required. This is because of the difficulty of assessing the existing (or background) concentrations. These are either (a) usually not known at the location of interest, (b) highly variable and may not have peaks at the same time as traffic-related peaks, or (c) may already include contributions from the transport project being assessed, resulting in double counting. A full assessment of air quality effects against the Standards, or other criteria, is left to Tier 3 assessments.

7.4.2 Derivation of significance criteria

The recommended approach for Tier 2 is to determine whether the quantitatively derived adverse effects are likely to be 'significant'. This is an absolute criterion that is not related to the existing air quality.

The concept of significance allows for a level of increase in the air pollution due to a transport emission source, as long as the maximum increase is below some specified significance criterion for each pollutant being assessed. (See also the discussion on 'reasonableness' in section 4.6.) Table 7.3 recommends criteria for determining whether effects are likely to be significant based on such an incremental analysis. These criteria are suggested as a guide only. The criteria are necessarily stringent because they are intended to be applied in any location, regardless of existing air quality.

The selection of appropriate criteria for significance is based to some extent on the relative effects of the increase, and to some extent on the inherent variability in the measurements due to space and time variability in the concentration of the pollutant. These assessment criteria are somewhat subjective, but are based on practical considerations. The recommended criteria are tighter for longer time averages because of the serious health effects associated with long-term exposures to PM10 and NO2.

NO2 one-hour: This is set at 20 µg/m3, or 10% of the national ambient air quality standard. This is around the same order as the background value found in many houses (those with electric heating - it can be over 100 µg/m3 for those with gas, wood or coal heating) and is lower than that found inside most vehicles. The health effects of short-term increases of this order will be very small.

CO eight-hour: This is set at 1 mg/m3, or 10% of the national ambient air quality standard. Short-term exposures to CO tend to be reversible. Exposure to elevated values over periods longer than eight hours may have some long-term effects, as may exposure to very high levels (greater than 10 mg/m3). The general background through any large city tends be around 1−2 mg/m3.

PM10 and PM2.5 24-hour and NO2 24-hour: The thresholds are set at 5% of the national ambient air quality standard and/or guidelines (where standards do not apply). Longer-term exposure to these pollutants has a more serious health effect, and is consequently set at a relatively low percentage.

These thresholds are not intended to define an acceptable or insignificant increase in actual ambient air quality. Instead, they are intended to provide an indication of the likely significance of predicted ground-level concentrations. These thresholds are suggested in the context of a Tier 2 assessment, which is conservative and based on worst-case scenarios.

7.5 Reporting a Tier 2 assessment

The results of a Tier 2 assessment should be documented for inclusion in any assessment of environmental effects, and to provide the basis for a Tier 3 assessment, where necessary. The report should summarise the findings of the Tier 2 assessment, including the basis of the traffic information, air quality information, any assumptions and their justification. Recommended reporting requirements are given in section 4.5.

Tier 2 screening assessment − recommendations

A Tier 2 assessment is a relatively quick and easy screening exercise. Any users who are not familiar with the assessment process should read this section in conjunction with section 8.

A Tier 2 assessment should be undertaken for all locations identified in the Tier 1 assessment, or for the worst-case locations. The assessment should be undertaken both with and without the proposal. The air quality assessor must understand the basis, meaning and limitations of any traffic data.

Consultation with the regional council is recommended before finalising any detailed Tier 2 assessment within an airshed that already exceeds the national ambient air quality standards.

4 Exceedances of the standard are required to be published by the council in local newspapers.