10 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 on potential mitigation measures is therefore provided below.
10.1 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, different mix of road/rail)
- carefully siting and sizing 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 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.
Other options, such as relocating properties or placing operational restrictions on sections of roads (similar to on-ramp signals), are not within the scope of the consent process but may be considered as useful alternatives.
It is important potential air quality effects be considered 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 any air quality solution in isolation.
10.2 Offsets
Measures to offset any overall increase in emissions may 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.
10.3 Vehicle emissions reduction
An alternative to offsetting emissions from other sources is to introduce options 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 significant impacts 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 more than 30 percent at a cost of $39,000/tonne of PM10 emitted (Auckland Regional Council 2005b).
- A public transport project could achieve a significant modal shift, 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.
