2. What is a CNMA?
In line with the aim of Article 1 of the Directive, a prioritisation matrix has been developed to facilitate a common approach to avoiding, preventing or reducing, on a prioritised basis, the harmful effects due to environmental noise exposure. The Scottish Government has developed this prioritisation based on the areas where people are most likely to be annoyed by noise from the sources defined within the Directive 1.
The objective of the prioritisation matrix is to identify areas where people living within these areas are most likely to be annoyed by noise from either road or railway traffic noise sources. The identification of such areas has been based on a scoring system which takes into account the number of people potentially affected, and the annoyance response to the particular noise source under consideration (either rail or road). The derivation and details of the Prioritisation Matrix is fully explained in Appendix 1 of this document. It is important to note that at this stage in the Action Planning process it has been decided by the Scottish Government Working Groups, through consultation with SEPA and the relevant local authorities, that an industrial noise source or an area affected by industrial noise should not be included in the prioritisation matrix and that any prioritisation, or noise intervention, of such industrial areas/sources should be at the request of the regulatory authority.
A statistical analysis of the calculated prioritisation scores was undertaken and a consensus was reached to identify the top three percent (3%) of each of the road and rail networks within each of the Glasgow and Edinburgh agglomerations and the qualifying 2 major roads and railways outwith the two main agglomerations. The top 3% of calculated scores were then subdivided into 1% bands and it has been agreed by the Scottish Government Working Groups, endorsed through public consultation, that the top 1% in each of the agglomerations and areas affected by qualifying road and rail links outwith the two main agglomerations are to be identified as CNMAs.
It should be remembered that, in general, each CNMA was determined from a 100m length of line noise source (rail or road) and the prioritisation process. Therefore, it is important to appreciate that the aim of CNMAs is to identify the most likely areas where the highest population noise exposures occur and, as such, it may be appropriate to reduce or extend the coverage of these areas where local conditions warrant changes in CNMA coverage. Examples where this may occur are: where there is a series of terraced houses, it would be inappropriate to restrict consideration to just those properties that lie along a 100m stretch of road if there are a small number of properties that lie outwith the existing CNMA coverage; conversely, if the CNMA coverage simply takes in one or two tall flatted buildings then it may be more appropriate to reduce the extent of the CNMA. Stakeholders will be required to exercise careful judgment in this regard.