A critical review of the method used for the assessment of traffic noise impact for major road schemes in Ireland.


Author : Diarmuid Keaney MSc., MIOA



As part of all major Irish road schemes, an Environmental Impact Statement (EIS) is routinely prepared, addressing a number of environmental considerations.

This present study relates to the environmental consideration of traffic noise in such EIS’s and how the noise impact of proposed roads through rural locations is assessed.  It critically reviews the methodology prescribed by the Irish National Roads Authority (NRA) for all national road schemes, with a focus on its application in rural Ireland.

Using an EIS already prepared for the proposed Galway City Outer Bypass (GCOB), a project which is ‘on hold’ at the time of writing, it was possible to study the data and methodology used at one rural location to build a project case study at Forramoyle West, a rural location on the west side of Galway city which is on the proposed route for the GCOB.

This study critically reviews the application in the EIS of the Calculation of Road Traffic Noise (CRTN) methodology and concludes that it is being incorrectly applied and misused.  It also closely examines the rural baseline noise climate at Forramoyle West and highlights a number of perceived shortfalls in the NRA methodology and its application to rural locations.

The study also examines the use of a derived Lden index and reviews the Transport Research Laboratory’s (TRL) conversion methodology and the NRA’s adaptation of this methodology.  The intended purpose of the TRL conversion methodology is considered, as is the manner in which the NRA have adapted it for their guidance for the assessment of noise for major road schemes in Ireland.

It is further argued that the NRA guidance and design goal (Lden=60dB) does not give adequate consideration to the prevailing noise climate in rural environments. This is contrasted with a widely used and – in the author’s opinion – more appropriate method of assessing noise impact which is detailed in the Design Manual for Roads and Bridges (DMRB) (Highways Agency UK, 1994).

Finally, the case is presented that the NRA guidance document prescribes a methodology which is not appropriate for rural baseline noise studies.

In summary, this study offers a number of conclusions which, in the author’s opinion, highlight the need for a complete review of the NRA guidance document as well as a review of the design targets for future EU compliance.


Study Conclusions

This study describes a critical review of the methods used for the assessment of traffic noise impact for major road schemes in rural Ireland.  Having reviewed supporting documentation used in the development of the NRA guidelines (NRA, 25th October 2004) as well as researching additional relevant and related documentation for the assessment of traffic noise impact, the study draws the following conclusions:

1) The shortened CRTN methodology, which the NRA guidance document prescribes for the measurement of baseline noise, is not applicable to Irish rural locations, in the author’s opinion.  It should also be noted that a large number of Irish road schemes are in fact “bypass road schemes” which are typically routed through rural locations.

2) A review of one baseline location (Location 15E) on the GCOB EIS, has, in the author’s opinion, highlighted a number of errors, omissions and shortfalls relating to noise measurements and the application of the CRTN methodology which has been used to derive the baseline Lden.  Research would indicate that the L10 index is intended for measurement of traffic noise.  However, the project case study indicates that at rural locations it is being used for measurement of ambient ‘non-traffic’ noise and hence is not valid for conversion to traffic-related indices.

3) Research shows that the TRL (LA10,18hour conversion to Lden) conversion was an interim action for the conversion of the widely used LA10,18hour in the UK for the purposes of noise mapping only, to allow the UK to comply with EU noise mapping directive 2002/49/EC. In the author’s opinion then, the NRA guidance documents prescribe an adapted version of the TRL conversion methodology for an application for which it was never intended.

4) The Design Manual for Roads & Bridges (DMRB) (Highways Agency UK, 1994), Volume 11, Section 3, Part 7 on Traffic Noise and Vibration, provides an ideal methodology for the assessment of “Nuisance from Traffic Noise”.  The NRA guidance (NRA, 25th October 2004 p. 41) shows familiarity of this document, however it appears to have overlooked the guidance offered in the DMRB Chapter 5, titled “Measuring and Predicting Noise Levels and Assessing Noise Nuisance”.  This omission is regrettable, as it would allow the “Do Nothing” and “Do Something” scenarios to be assessed and expressed as a change in % nuisance.

5) The review of the NRA validation study has, according to the author’s reading, identified a number of shortfalls in the research and potentially an over-simplification of the empirical relationship between measured LA10,18hour and Lden with a “one-fits-all” equation.  It is the author’s opinion that the accuracy of this “one-fits-all” approach is questionable, but unfortunately it appears to be enshrined into the NRA guidance (NRA, 25th October 2004).

6) Finally, it is also the author’s opinion that the NRA design goal of 60dB does not give adequate consideration to rural noise climates; this view is supported in the present study by assessment using recognised alternative guidance (DMRB). The author further argues that the NRA design goal is somewhat less onerous than is claimed in the NRA guidance document and should be reviewed, giving consideration to future EU requirements and the necessity to include reporting of the Lnight index.


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