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Discusses measures for reducing (mitigating) road traffic noise, including barriers, low noise road surfaces, and building acoustic treatments, and how and when the Transport Agency implements these measures. Also described is noise monitoring and assessment, and the circumstances under which noise monitoring is undertaken.

  • The road-traffic noise at my house is very loud, will you monitor noise?

    The Transport Agency does not typically monitor noise at individual properties in response to complaints or queries about road-traffic noise. 

    This is primarily because specialised and long-duration monitoring (days to weeks) is required to measure road-traffic noise.  And even then, it can be difficult to reduce the influence of other environmental noise (e.g. wind and other weather, human activities, bird calls etc.) and capture typical traffic conditions. This is also not a sustainable process across the more than 11,000 km of state highways in New Zealand.

    Even where noise monitoring may be practical and accurate, there are generally no standards or criteria for comparison of noise monitoring results.  Therefore, the monitoring information only has limited use.

    For road-traffic noise assessments for new roads and material alterations to existing roads noise models are used.  Long-term monitoring is sometimes undertaken to verify the noise model, but noise measurements are not used to confirm compliance with project requirements at individual locations.

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  • I have measured noise with my mobile phone. Is it accurate?

    Some mobile phones provide reasonably accurate noise measurements, especially in still conditions. However, because mobile phone microphones do not have windscreens (the black foam seen on many microphones) they are subject to wind noise, which can cause high noise readings. Also, mobile phone microphones are not calibrated so readings can be systematically too high or low. 

    Another consideration is most noise criteria in New Zealand are averaged over time. For road-traffic noise the average is for a 24-hour period and for construction noise the average is generally 15 minutes to an hour. While it may be practical to estimate construction noise with a mobile phone, it is unlikely a mobile phone could be used to measure road-traffic noise over a 24-hour period.

    As mobile phone noise measurements are not taken using an approved and calibrated device as required by New Zealand Noise Standards, reliance cannot be placed on the readings.

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  • Why doesn’t the Transport Agency monitor noise at houses located far from the road?

    The Transport Agency does not typically monitor noise at individual properties to address complaints, concerns or compliance. 

    This is particularly the case for houses located away from the road (i.e. greater than 50m to 100m from the road in urban areas and greater than 100m to 200m from the road in rural areas). 

    This is because in these locations, noise from the road may not be ‘dominant’. Dominant means noise levels are significantly above background (residual sound) levels. Background noise levels also include noise from wind and other weather, bird calls, human activities (e.g. talking, lawn mowing, use of a car, heat pump etc.) or noise from local roads. 

    In locations where the road noise is not dominant, it does not mean you will not be able to hear noise from the road. In fact, you might be able to hear noise from the road clearly -  especially at night.

    Noise monitoring measures the total noise at a location from all sources. When noise from background/ambient noise sources is dominant or similar to noise from the road it can be difficult to attribute noise to a specific source. 

    For example, if the background (residential sound level) noise level is 40 dB and the noise level from the road is 55 dB then the total noise level is 55.1 dB (the logarithmic sum of 40 and 55 dB). In this example, the road noise is dominant because the background noise has only a small influence on the total noise level and it is relatively easy to attribute the noise level to the road. 

    As the ‘road’ and ‘background’ noise levels get closer to each other, the influence of the background noise level on total noise level (background plus road) in decibels is greater. For example, if both the background and road noise levels are 40 dB the total noise level is 43 dB (the logarithmic sum of 40dB and 40dB). In this example, it is more difficult and unreliable to differentiate the contribution of noise from the road and from background.

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  • How does the Transport Agency predict noise exposure across the state highway network?

    The Transport Agency is in the process of preparing a model of noise exposure across the state highway network.  It is expected this model will be complete in 2019.

    This model will identify areas of potential ‘high’ noise exposure based on information such as traffic data, road surfacing information, location of building (house) footprints and ground level in relation to the road.

    Across the state highway network terrain and house location information is available in inconsistent detail. For example, better terrain and house location information is available for many cities but less detailed information available for some rural areas. This means the model is not sufficiently accurate to reliably predict noise exposure at all individual houses but is sufficient to identify areas of potential high noise exposure.

    Due to potential privacy concerns and the limitations of the model, the NZ Transport Agency does not share information about noise exposure at individual properties or houses.

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  • How do noise barriers work?

    Sound (noise) travels in waves but may be thought of as travelling as a ray.  Sound waves can be absorbed, reflected or diffracted (bent) by noise barriers (solid walls, fences and earthen bunds).  Absorption, reflection and diffraction all reduce the energy of sound and thus the noise level, which is commonly measured in decibels.

    Level ground with noise barrier

    Level ground with noise barrier.

    The straight sound path is usually the most significant and by introducing a barrier between  the source and the receiver (e.g. a dwelling), the amount of sound reaching the receiver can be reduced. While reflected noise and diffracted noise may reach the receiver - as noted above - the overall noise levels will be reduced. 

    To provide the most noise reduction barriers should block line of sight from all parts of a dwelling to all points of a road and to all vehicles using the road. 

    We have prepared a guide for noise barriers, which is available on our website. The intended audience for this guide is primarily engineers, architects and contractors, but may be of general interest to homeowners and developers.

    State highway noise barrier design guide [PDF, 3.7 MB]

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  • What is the best location for a noise barrier?

    The best location for a noise barrier depends on the position of the house in relation to the road. 

    Barriers should ideally block line of sight from all parts of a dwelling (if that is not practical then from the midpoint of windows) to all points of a road. In practice this may require a very tall and/or very long wall that could be impractical to construct.

    Noise barrier length

    Noise barrier length.

    Noise barrier with return

    Noise barrier with return.

    In general, the best location for a noise barrier is as close to the road as possible when houses are located below the road. 

    Level ground with noise barrier

    Level ground with noise barrier.

    When houses are located at the same level or above the road the most effective barrier location may be as close as practical to the house.

    Noise barrier located at top of cutting

    Noise barrier located at top of cutting.

    Care needs to be taken during the design and location of barriers to consider urban design effects and on-going maintenance requirements, in addition to district plan requirements.  Other effects of barriers which should be considered include shading, visual effects, access, safety and surface/storm water flow. 

    The Transport Agency does not provide noise barriers on privately owned land to reduce noise from existing roads. 

    We have prepared a guide for noise barriers, which is available on our website.  This guide provides more details on the factors that influence noise barrier details and siting.

    State highway noise barrier design guide [PDF, 3.7 MB]

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