Low-noise asphalt will be used along most of the new motorway. This is a quieter surface than the chipseal used on many roads. Noise reducing earth bunds, fences or concrete barriers will also be used where possible in areas near housing.
It is believed that these mitigations will keep noise to an acceptable level. Testing and modelling were used to measure and predict noise levels in the area before any changes were made, and these findings have been used to design the noise mitigation for each area of the project. The NZ Standard for Road traffic noise – new and altered roads (NZS 6806:2010) is being used.
All the noise mitigations required for this project are in place except for the final asphalt layer on the road. The asphalt surface, that is significantly quieter than a chip seal surface, cannot be laid until the next sealing season (spring) after the road has had a chance to weather, this is in line with industry best practice. This is in line with industry best practice.
In response to concerns around traffic noise, we have done some additional early monitoring, before the final low-noise asphalt is laid. We measured the sound levels in seven locations along the project with results between 54-61 deciBels (dB). These early results show the mitigations already in place and required by the resource consent, ie the bund, fences and concrete barriers are working as expected, and once the low noise asphalt is applied, we expect this level will drop another 5 to 7dB. We are confident that the final low noise asphalt will be effective at bringing the noise levels under that required by the NZ Standard for road noise near residential properties (NZS 6806:2010) which for a new road is 57 dB (averaged over 24 hours) or less. This represents a significant decrease in noise which should be very noticeable for residents.
Will the noise level increase as traffic numbers increase?
Noise increases with vehicle numbers, that’s why the mitigations for the motorway were designed with a higher number of vehicles in mind (we are expecting about 35,000 vehicles in a decade or so). We are confident that the CNC motorway will still meet the noise levels predicted by the modelling with increased traffic on it, when all the noise mitigation measures are in place (including the asphalt layer).
Could you plant more trees or dense vegetation to reduce the noise?
Trees and shrubs indeed provide a psychological impression of less noise but are not the most effective barriers. Dense vegetation makes little difference. Sound experts will recommend impermeable barriers of concrete or wood, they deflect noise much better than for instance hedges. The CNC urban design landscape plan is actually not part of the noise mitigation measures, it may over years contribute a little, but the earth bund with timber fences, the concrete barriers and the special asphalt will make the noticeable difference.
Can you reduce the speed limit on the motorway to 80km/h to reduce the noise?
Decreasing the speed limit will reduce noise. However, in order to be effective a substantial reduction is necessary. Reducing the speed limit to 80km/h would only reduce the noise leave by 1 to 2dB and this is not noticeable to most people. The Christchurch Northern Corridor is designed, constructed and maintained at 100km/h and the speed limit for heavy vehicles and vehicles towing remains at 90km/h. There would be little to no benefit in reducing this speed limit. The motorway with a speed of 100km/h provides for several benefits: reducing congestion, reducing travel times and improving productivity (net economic benefit).
What about the gap in the fencing at Owen Mitchell Park?
Noise mitigation measures work best when: they are close to the source of the noise - like the concrete barriers or close to housing like to the earth bund and fencing. There is a gap in the bund and fencing at Owen Mitchell Park to allow for safe access to and from the park and the shared path on the CNC. As there is no housing near this gap in the bund and fencing, it is not creating an area where housing will have a noise level over the traffic noise standard.