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9 Mitigation Options

Depending on the proposal, mitigation measures may need to be considered in any assessment of the effects of land transport on air quality. Some general discussion of potential mitigation measures is therefore provided below.

9.1 Measures to mitigate local impacts

There are a number of measures to mitigate localised air quality impacts from transport projects (or projects that affect traffic), which can be implemented by those responsible for the projects or affected by the design of the project. Some examples are:

  • increasing the distance between the road and receptors, particularly at intersections

  • changing land use adjacent to transport corridors

  • alternative transport schemes (eg, a different mix of road and rail)

  • carefully siting and sizing of tunnel exhaust vents or chimneys

  • traffic management systems

  • flow restrictions

  • orientation of the road in relation to prevailing winds

  • measures to offset any increase in emissions or reduce vehicle emissions (see below)

  • indirect measures that may influence travel demand, fleet composition, vehicle types and fuel quality.

If the assessment shows there will be locations where people will be exposed to air pollution levels that exceed the national ambient air quality standards, then mitigation will very likely be required. Because contaminants generally disperse quickly with distance from their source, separation is likely to be one of the most effective mitigation measures. Options to achieve adequate separation could include:

  • moving the road alignment away from receptors

  • placing part of the road in a ventilated tunnel

  • relocating properties

  • placing operational restrictions on sections of roads (similar to on-ramp signals).

Another mitigation option is to introduce measures that will reduce overall emissions from the project. Here are some examples.

  • Bus priority lanes or freight priority lanes could improve average speeds, which may reduce emissions from buses or trucks. The majority of PM10 emissions are from heavy-duty diesel vehicles, so this could have a significant impact on overall emissions.

  • Similarly, targeted measures to reduce emissions from buses or trucks could have significant impacts. Auckland Regional Council analysis has shown that a catalyst retrofit scheme for older buses could reduce emissions by more than 30% at a cost of $39,000/tonne of PM10 emitted (Auckland Regional Council, 2005b).

  • A public transport project could achieve a significant modal shift by, for instance, having more people travel to work by bus rather than using their own vehicles, potentially resulting in an overall reduction in emissions.

  • Other travel demand measures could affect the link under study; for instance, the opening or closing of a facility that attracts people using vehicles, such as a supermarket.

  • Minimum emissions standards could be set through local bylaws (as is commonly used for vehicle tonnage restrictions).

It is important to consider potential air quality effects when a transport development is in the planning stages, particularly if traffic passes near sensitive receptors such as schools, childcare facilities or hospitals. It is also important to note that mitigation measures to reduce effects from air discharges at a local level may in turn influence other aspects of the proposal (eg, noise, road safety, discharges to water). In developing mitigation measures for a land transport development, all these aspects must be considered in combination rather than pursuing any air quality solution in isolation.

9.2 Offsets

Measures to offset any overall increase in emissions may sometimes be the only realistic mitigation option, particularly when existing air quality is relatively poor. For projects that result in a net increase in emissions, it is recommended that at least the equivalent of the emissions from other sources be offset before proceeding with the transport project. Some offsets may also be necessary if emissions from the project are sufficient to cause unacceptable localised impacts when added to (relatively high) background levels.