A8.2 - Road traffic noise

Road traffic noise

Noise is a disturbing or otherwise unwelcome sound, which is transmitted as a longitudinal pressure wave through the air or other medium as the result of the physical vibration of a source. Noise propagation is affected by wind and intervening absorbing and reflecting surfaces, and is attenuated with distance.

Road traffic noise sources include:

• engine and transmission vibration
• exhaust systems
• bodywork and load rattle
• air brake and friction brakes
• tyre/road surface contact
• horns, doors slamming, car audio systems
• aerodynamic noise

Impacts of road traffic noise

Road traffic noise is generally continuous, and long term exposure can have significant adverse effects. These can be categorised as disruptive impacts, such as sleep disturbance and speech interference, and psychological impacts such as annoyance reaction and other behavioural impacts. While there is no evidence of permanent hearing loss from road traffic noise, there is a great deal of evidence to show that noise can cause adverse health effects in people due mainly to stress-related factors.

While the untrained ear will generally only detect noise level differences of 3 decibels (dB) or more, smaller increases will still affect peoples well being. To increase the noise level by 3 dB requires a doubling of traffic volume.

Design guidelines for road traffic noise on State Highways

Design guidelines for the management of road traffic noise on state highways are given in Transit New Zealand's Guidelines for the management of road traffic noise - state highway improvements. These guidelines apply to noise-sensitive facilities adjacent to either new state highway alignments or to any other State Highway improvements, which require a new designation.

The assessment point at which the design criteria apply is one metre in front of the most exposed point on the façades of existing residential buildings or educational facilities. An exception is in the case of noise buffer strips where the assessment point is the outer limit of the buffer strip.

The two criteria in the guidelines, both of which apply, are:

1. Average noise design criteria
   
   The average noise design levels for residential buildings and educational facilities at the assessment point are set out in table A8.1.
   
   If it is not practicable or cost effective to meet the average design noise criterion at the assessment point given in table A8.1, then the guidelines specify internal noise design criteria. These criteria apply to all living rooms (including kitchens) and bedrooms in residential buildings, or teaching areas in educational facilities, with windows closed on the exposed walls.
   
   The internal noise level criterion for residential buildings is either the level given in table A8.1 minus 20 dB(A), or 40 dB(A) Leq (24 hour), and for educational facilities the internal noise level criterion is either the level given in table A8.1 minus 20 dB(A), or 42 dB(A) Leq (24 hour), in each case whichever is the higher.

Table A8.1 Average noise design levels (leq (24 hour))

(b) Single noise event design criterion

A single noise event is the maximum noise level emitted by a single vehicle passing the assessment point.

Where the assessment point for residential buildings and educational facilities is less than 12 metres from the nearside edge of the traffic lane, the Transit Guidelines for the measurement of road traffic noise - state highway improvements require noise reduction measures to reduce noise by at least 3 dB(A). This is designed to provide a level of protection to properties from the noise effects of single vehicles.

Mitigation of road traffic noise impacts

There are various options for reducing the effects of road traffic noise. These include realignment to increase the distance between the roadway and the assessment points, noise buffer strips, barriers, alternative road surfaces (Dravitzki et al 2002 and 2004) and building insulation.

Where project optimisation requires noise mitigation measures, the cost of such measures will be identified and included in the project cost as discussed in chapter 3.

Measurement and prediction of road traffic noise impacts

Traffic volumes used for noise predictions shall be based on forecasts of traffic flow 10 years after the completion of the project.

Equipment and methods for the measurement of noise shall comply with NZS 6801: 1991 Measurement of sound. Prediction of road traffic noise shall be carried out using the United Kingdom Calculation of road traffic noise (1988) method, calibrated to New Zealand conditions (refer to Transit NZ Research report 28, Traffic noise from uninterrupted traffic flow (1994)) and converted to the appropriate Leq index.

The conversion formulae to calculated Leq values from the L10 values derived from the UK Calculation of road traffic noise (1988) method are:

Leq (24 hour) = L10 (18 hour) - 3 dB(A)

Leq (1 hour) = L10 (1 hour) - 3 dB(A)

Valuation of road traffic noise impacts

There have been no specific studies carried out in New Zealand to determine the cost of road traffic noise however there is evidence to suggest that road traffic noise levels of 53 to 62 dBA do encourage people to move out of an area more quickly (Dravitzki et al, 2001).

A British survey (1995) of international (predominantly hedonic price) valuations suggests that the costs of noise are approximately 0.7% of affected property values per dB. A Canadian survey (Bein 1996) found that hedonic pricing revealed typical costs of 0.6% of affected property prices per dB, and the OECD recommends noise valuation based on 0.5% per dB. Bein argues that the total costs of noise are much higher than the change in property values because:

• consumers may not consider the full effects at time of purchase (supported by a German study which showed increased willingness to pay with increased understanding of noise);
• effects on other travellers and on occupants of commercial or institutional buildings are not captured;
• hedonic studies typically consider values of homes which experience noise above and below certain levels (a German study shows increasing willingness to pay as base noise rises).

A reasonable figure for New Zealand is suggested as being 1.2% of value of properties affected per dB of noise increase, (0.6% multiplied by a factor of two to take into account the factors mentioned by Bein). Using the median house price of $327,000 (Real Estate Institute of NZ, 12 months to June 2007) and occupancy of 2.6 persons, this suggests a NPV cost of $3,924 per dB per property and $1,500 per dB per resident affected ($410 per household or $160 per person per year). This figure should be applied in all areas, since there is no reason to suppose that noise is less annoying to those in areas with low house prices. It is arguable as to what range of noise increase the cost should be applied to, but a conservative approach would be to apply it to any increase above existing ambient noise. This reflects a belief that most people dislike noise increases, even if the resulting noise is less than 50 dB.

Costs of road noise shall be incorporated into the external impact valuation (worksheet A8.1) and valued at:

$410 per year × dB change × number of households affected.

Where noise affects schools, hospitals, high concentrations of pedestrians and other sensitive situations an analysis may be required to determine the cost of noise that is site specific. The methodology for undertaking a valuation of noise at sensitive sites should be appropriate to the site (ie, willingness to pay surveys may be appropriate for sites with high concentrations of pedestrians and inappropriate for hospital sites).

Reporting of road traffic noise impacts

The number of residential dwellings and the educational facilities affected by a change in road traffic noise exposure shall be reported in terms of:

1. the predicted change from the ambient noise level
2. the difference between the predicted noise level and average noise design levels given in table A8.1.

Predicted noise levels, which exceed the design guidelines given in Transit New Zealand's Guidelines for the management of road traffic noise - State Highway improvements, shall be reported on the worksheet A8.3.

Where noise is a significant issue, plans shall be prepared distinguishing each type of land use. These plans shall show:

1. contours of noise exposure in the do-minimum and for each project option, and changes in noise exposure in bands of 3 dB(A), ie, 0 to 3 dB(A), > 3 to 6 dB(A), > 6 to 9 dB(A)
2. the number of residents in each band
3. where the predicted noise level is above the average noise design levels given in table A8.1 or where the single event criterion should apply.

Where projects incorporate measures to mitigate noise, the incremental costs and benefits of these measures shall be reported. If appropriate these costs and benefits shall be reported for various levels of noise mitigation.