Proceedings of the Institute of Acoustics

 

 

Health and whole life cost benefits of highways noise barriers

 

Giles Parker¹, Sound Barrier Solutions Ltd, Ansty, Coventry, UK

 

ABSTRACT

 

As with any asset on the highways network, it is common sense to allow for realistic maintenance costs when choosing and procuring a noise barrier system. A short-term approach of focusing on newly installed prices alone creates a false economy and potentially stores up financial hardship for the future. The adverse impact of road traffic noise on public health is also long appreciated. Being able to quantify in financial terms not the only perceived annoyance but also the long-term damage to health due to traffic noise helps to justify why one particular noise barrier design will benefit the community more than another over its life cycle.

 

1. MAINTENANCE FREE?

 

The Design Manual for Roads and Bridges (DMRB) document CD 355 covers the application of whole-life cost for design and maintenance of highways structures. Appendix B gives an example of a life-cycle appraisal for a highways structure stating that a 60-year period is normally required.

 

Few noise barrier systems currently installed on the UK network can be considered maintenance-free and are certainly not designed for a 60-year life. At the very least, any highways noise barrier designer/specifier should be providing a comparative cost appraisal for the life cycle of the noise barrier that includes for – the initial installed cost – PLUS the full reinstallation cost PLUS the clearance and removal cost of the existing degraded system PLUS any traffic management cost associated with the replacement – each time the barrier needs replacing.

 

This is apart from any localized repair work that might be required on an ad hoc basis, not to mention the impact on road traffic flows or journey time reliability each time a replacement is required.

 

2. FURTHER GUIDANCE

 

DMRB - GD304 – Designing health and safety into maintenance considers the safety benefits of choosing assets with a higher design life. As an example, it states in Table E/A.1 ‘choose barrier design life taking into account both working life costs and the safety issues associated with in service maintenance of the system’

 

DMRB - LD119 - Roadside environmental mitigation and enhancement - Chapter 5: Noise Barrier Design – para 5.9 requires that noise barriers ‘have a non-acoustic durability of at least 20 years. By the 60th year of its life cycle, such a noise barrier could be being installed for the 4th time having

 

required full replacement 3 times already! Will the specifier take into account the whole life costs associated with all these anticipated replacements?

 

DMRB – CD354 – Design of minor structures - Chapter 5: Design – para 5.5 requires that design life of minor structures including environmental (noise) barriers to be 25 years – during which time only very minor repairs would be expected if any.

 

When such barriers degrade so fast it becomes essential to financially quantify the effects of durability when choosing between noise barrier systems based on their whole life cost benefits and long-term acoustic performance. The Design Manual for Roads and Bridges (DMRB) document CD 355 covers the application of whole-life cost for design and maintenance of highways structures. Appendix B gives an example of a life-cycle appraisal for a highways structure stating that a 60-year period is normally required.

 

3. TIMBER ABSORPTIVE BARRIERS

 

Following rigorous testing of installed barrier systems, the Transport Research Laboratory published a project report – PPR490 on the Acoustic Durability of timber noise barriers on England’s strategic road network. The test results suggest that the acoustic performance of timber absorptive barrier degrade in acoustic performance by approximately 7dB after only 5 years. Over the same time-frame single-skin timber reflective barriers to degrade by the order of 4-7dB but starting from a much lower initial sound insulation level.

 

GEOTEXTILE MEMBRANES

 

form an integral part of standard UK timber absorptive barriers designs, having both a non-acoustic and an acoustic purpose. Their primary non-acoustic purpose is as a protective and supportive membrane over the absorptive mattress to hold it in place and reduce its deterioration. Deterioration of the geotextile will have an integral impact when it becomes brittle, fractured and failed. This has been observed to occur in situ after a relatively short time ~5-10 years.

 

According to geotextile manufacturers non-pigmented geotextiles with UV stabiliser give ~ 5-year predicted life. High performance carbon black polypropylene give ~ 10 years of life. After this the inherent performance of the geotextile would be compromised. This contrasts markedly with the requirements of LD119 (20 years non-acoustic) and CD354 (25-year design life).

 

In the harmonised standard for highways noise barriers, the standard for non-acoustic durability of highways noise barriers; EN 14389-2:2004 states in the Introduction “All elements in the construction of noise reducing devices should be resistant to corrosion and embrittlement, be dimensionally stable and generally have a high ageing resistance in many differing conditions”.

 

When noise barriers degrade so fast it becomes essential to financially quantify the effects of durability when choosing between noise barrier systems based on their whole life cost benefits and long-term acoustic performance.

 

4. TAG WORKBOOK

 

The adverse impact of road traffic noise on public health is long appreciated. Being able to quantify in financial terms not the only perceived annoyance but also the long-term damage to health due to traffic noise helps to justify why one particular noise barrier design will benefit the community more than another over its life cycle.

 

DEFRA has produced guidance on assessing the impacts of transport-related noise using an ‘impact pathway’ approach and covering a range of impacts on: annoyance, sleep disturbance, and health

impacts, including heart disease (acute myocardial infraction, or AMI) stress and dementia. The TAG Noise Workbook in the WEBTAG appraisal analysis provides a way of determining the impact of highway noise on these health aspects and compares the overall (holistic) cost benefit over a 60- year life cycle or different mitigation measures such as alternative noise barrier designs.

 

5. NOISE MODELLING

 

To help illustrate how the health and whole-life cost benefits of noise barriers might be compared, a hypothetical road traffic noise model was produced using CadnaA to predict how noise propagates from a dual carriageway towards a nominal 300-house residential scheme.

 

 

Figure 1: Hypothetical (CadnaA) 300-house road noise model

 

The houses were spread over a 600m length adjacent to one carriageway, each house with a receiver on its most exposed first floor façade. To be generic, the model was assumed flat.

 

Using CRTN, different barrier design scenarios and types were compared to determine the average noise reductions due to barriers ranging from 600 – 1000m long and from 2 – 6m in height. The exercise was then repeated assuming 300 houses on either side of the carriageway for parallel noise barriers of the same dimensions.

 

Based on the health aspects listed above, the TAG Noise Workbook in the WEBTAG analysis can determine the financial health benefit of these noise reductions for different barrier heights, lengths and configurations. This financial health benefit is referred to as the Net Present Value. If TAG assumes that these noise reductions are maintained over a 60-year life cycle, then this maintenance will obviously require some barrier types to be replaced a lot more frequently than others.

 

By selecting different barrier types and material, a detailed cost comparison could then be made, knowing the typical installed costs of each barrier type, the expected number of replacements over a 60-year cycle to maintain performance, the cost of full reinstallation, the clearance and removal cost of the existing degraded system and any traffic management cost associated with the replacement. In so doing one can obtain a more realistic 60-year whole life cost comparison for different barrier scheme options.

 

The TAG Noise workbook then generates a ‘Benefit to Cost Ratio’ by dividing the Net present value by the whole life cost to determine which option offers the best long-term value to the country both acoustically and in health terms.

 

 

As an example, using the modelled performance of 800m long single barriers, a whole-life cost comparison was be made for a typical single-skinned timber reflective design and a timber absorptive design which tend to represent lower durability products and a physically and acoustically more durable system such as a metal absorptive barrier design.

 

CRTN is a blunt instrument and assumes that all the barriers give the same level of attenuation when new however when one takes into account the different maintenance expectations for each barrier type and includes for the overall cost of replacements over a 60-year life cycle to maintain that performance, the Benefit to Cost Ratios may differ enormously.

 

 

Figure 2: Benefit to cost ratio comparison for single noise barriers over 60 years

 

For every barrier height from 2-6m a more durable metal absorptive system would normally give the greatest benefit to cost ratio. For the 3m high barrier design it could be almost double that of the same timber absorptive over 60 years. It is also worth noticing in Table 1 that, because of its superior durability a 6-metre-high metal absorptive system would not only provide twice the acoustic noise reduction of the timber options and protect 60% more properties, it would do so whilst still maintaining a considerably greater benefit to cost ratio over a 60 year life-cycle than a 3-metre high timber absorptive system. Whilst the initial cost would be more, the long-term investment would prove considerably more worthwhile.

 

Table 1: Whole-life performance comparison of single noise barriers over 60-years

 

 

7. PARALLEL BARRIER COMPARISONS

 

The comparison difference is even more stark for the 800m long parallel barrier scenarios. Here metal absorptive barrier designs maintain a high benefit to cost ratio from heights of 2 to 6 metres whereas less durable timber options, though typically cheaper when first installed, demonstrate in the long run that they represent lower value for money to the country in terms of public health and the acoustic environment.

 

 

Figure 3: Benefit to cost ratio comparison for single noise barriers over 60 years

 

Again, building a taller durable barrier system is shown to give a much greater noise attenuation for far more people and a greater overall sustained health benefit than low-cost systems that may be more prone to rapid deterioration. Table 2 again shows that a 6-metre durable noise barrier system would prove in the long term a better long-term investment than lower less durable options, improving the life of residents and ultimately reducing the national health bill.

 

Table 2: Whole-life performance comparison of parallel noise barriers over 60-years

 

 

8. CONCLUSIONS - THINGS TO CONSIDER

 

Scenarios differ and road-to-residential schemes are not normally as uniform as this, however the principles remain true. There will be different barrier material types that are more durable than others, timber-concrete based absorptive barriers have been shown to be among the most durable systems both acoustically and structurally. Realistic assumptions have been made regarding the lifecycle of different barrier types and the costs associated with each complete replacement. So, whether you agree or disagree with some of the values in these comparisons, when you specify - ask yourself

 

How are you determining the whole life cost of a highways noise barrier?

 

How are your factoring in for the durability of road traffic noise barriers when you specify them?

 

Do you allow for realistic rates for maintenance, replacement, removal and traffic management?

 

Rail barriers will require a similar approach – What would be the cost of access each time a barrier needs removing and replacing and are those being considered when pricing for a long-term barrier scheme?

 

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¹ gilesparker@soundbarriersolutions.com

² Properties Benefitting: Those properties for which the façade noise levels reduced by 3dB or more