For Information

Provisional Legislative Council
Panel on Environmental Affairs

Assessment of the Effectiveness of Noise Barriers

Background

1.It is an established policy to require proponents of road projects to assess the potential traffic noise impacts from new roads through the process of Environmental Impact Assessment (EIA). The EIA will identify suitable road alignment and recommend the best practicable package of noise mitigation measures on the roads to meet the planning guidelines. In case there are residual impacts after adoption of the best practicable noise mitigation measures on the roads, acoustic insulation and air conditioning are provided to redress the residual impacts on existing schools and residential dwellings.

2.Noise barrier is one common form of noise mitigation measures. The EIA process will evaluate relevant factors including road safety and space constraints, and recommend the best practicable location, height and length of barrier for specific situations.

Effectiveness of noise barriers

3.The noise level reaching the receiver depends on the source strength and the degree of sound attenuation achieved during its propagation from the source to the receiver. In simple terms, the noise will be reduced if the transmission path is blocked by a barrier. As a rule of thumb, a noise barrier is most effective when it blocks the line of sight of the receiver from vehicles travelling on the road. The performance of a noise barrier is governed by its physical parameters i.e. location, height, length and mass. In this regard, the predicted performance of a noise barrier is a well tested science.

4.In the planning of noise mitigation measures for new road projects, barriers are designed to give the protection required when the noise impacts from the traffic on the road reaches a certain capacity in the planning year. The prediction of traffic noise impacts is carried out using an internationally accepted methodology which takes into account the traffic characteristics as well as the geometric relationship between the source, receiver and any noise attenuating materials in the pathway such as noise barriers. Such an approach has been commonly used in other countries and the prediction methodology has been tested and found suitable for local application.

5.Whilst the noise reducing performance of a barrier can be predicted with a reasonable accuracy as it has been proven over a few decades of work in this field, it is not always possible to conclude from field measurement of noise levels alone, the effectiveness of a particular barrier for a particular noise sensitive receiver. This is because of a number of reasons. Firstly, the traffic parameters will most unlikely be built up to the steady state pattern predicted for the capacity scenario. Second, and more applicable in Hong Kong than in other western countries, the new road with the barriers is most likely to be an additional element in an existing network where the noise measurements could not categorically attribute the result to the noise from the vehicles on the new road alone. So many mathematical corrections to account for the varying traffic parameters have to be carried out so as to render the exercise rather not cost-effective and impracticable for most cases.

6.However, since the late 80's, government's commitment to reduce the traffic noise impact through better planning has led to an increasing number of noise barriers being built on new roads. With this added experience, we have progressively strengthened the monitoring and audit framework so that wherever it would be practicable, the project proponent would be required to assess upon completion the effectiveness of the measures. This follow-up examination will include proper documentation on verification of the various physical parameters of the road and its noise barriers, with relevant traffic noise measurements, after the opening of new roads.

Environmental Protection Department
October 1997