Appendix A: Noise reduction assessment

Noise reduction is an important ecosystem service provided by vegetation, and one of particular importance in the context of road schemes where vegetation can act in a crucial role to buffer communities against additional noise pollution.

Identifying vegetation that is providing noise reduction benefits is dependent on several factors including the height, density, width, and area. It is also important to establish the location of any vegetation in relation to the noise source and receptors (e.g., dwellings) benefiting from noise regulation. The approach to the assessment was thus to review the threshold parameters in relation to the characteristics of vegetation to provide a noise reduction benefit, use these to identify areas of vegetation providing a benefit (considering the location relative to noise receptors), and to see if the proposed scheme would impact any potential benefits as a result of changes to vegetation.

Table 8‑1 presents the outcomes of a literature review carried out to understand the required, physical characteristics of vegetation to provide a noise reduction benefit.

Table 8‑1: Outcomes of literature review into threshold vegetation characteristics for provisioning of noise reduction benefits

Reference	Notes	Figures on threshold vegetation height	Figures on threshold vegetation depth
Peng et al., 2014	The aim of this study was to review available literature as well as to obtain specific experimental data to provide a better understanding of noise transmission when significant vegetation is present. The excess attenuation of traffic noise through 10 to 20m of trees (tree spacing <0.5m) was found to be typically 2 to 3dB, relative to Calculation of Road Traffic Noise (CoRTN) standardised calculation procedures predictions.	Only trees were referred to in the capacity to mitigate against traffic noise.	10-20m depth
Van Renterghem et al., 2012	Tree belts could be effective in reducing road traffic noise, on condition that planting schemes are optimised and tree density is sufficiently high. Calculations showed that a 15m deep and 2.5m stem height tree belt planted at 1m average spacing with 0.11m diameter tree trunks was found to have a performance equivalent to a standard 1.5m high noise barrier.	2.5m height trees	15m depth tree belt
Ow & Ghosh, 2017	This study was carried out to determine the effect of roadside vegetation on the reduction of road traffic noise under varying planting intensities. Roadside vegetation ranging from minimal planting through to moderate and dense plantings were used. The results showed that the traffic noise was reduced by 50% when vegetation was enhanced from a minimal to moderate planting intensity, and no enhancement in noise reduction was observed as vegetation was further increased to a dense intensity. A 5 m depth of vegetation barrier was found to be an ideal depth for traffic noise reduction. This report also investigated the effectiveness associated with setbacks where it was found that the greater the setback distance, the higher the level of noise amelioration and a 10 m depth was identified as the threshold for an effective tree belt. On further investigation, the evidence in the paper suggests that while a 5m depth of general vegetation can provide some noise mitigation, a 10m depth for a tree belt is needed to achieve the best results.	Vegetation belts referred to are trees in the vast majority of instances	Moderate vegetation density required; 10m depth tree belt
Waters et al., 2023	Tree belts and forestry can provide sound reductions of up to 6 dBA for a 15m depth, where performance depends on the spacing between trees, trunk diameter and leaf size and density. Generally, the larger the leaf side, the greater level of sound reduction provided. To achieve similar results to a dedicated noise barrier, vegetation belts need to be 15m thick with a shrub layer and care must also be given to the plant placements.	Other vegetation types do have a noise buffering impact, but only trees are referred to with regards to a capacity to abate noise in a comparable manner to a noise barrier	Tree belt of 15m depth
Forest Research, 2025	Planting “noise buffers” composed of trees and shrubs can reduce noise by five to ten decibels for every 30 m width of woodland, especially sharp tones, and this reduces noise to the human ear by approximately 50%. It is recommended to ‘plant tall trees where possible' to maximise the effectiveness of the capacity for vegetation to act as a noise barrier.	Noise buffers should comprise 'trees and shrubs' (i.e. trees with an understorey)	30m depth
GreenBlue Urban, 2015	With regards to tree size, it has been proven that noise reduction tends to increase with tree height up to 10-12m, after which it tends to decrease. This is a result of lower branches dying and allowing sound to travel more easily.  Published results on the effectiveness of trees as sound barriers vary enormously, however, a study by Huddurt in 1990 shows that in some instances noise can he reduced by 6 dB over a distance of 30 meters where planting is particularly dense. Leonard and Parr (1970) and Reethof (1973) found that a dense belt of trees and shrubs between 15-30 m wide could reduce sound levels by as much as 6-8 dB. Cook and Van Haverheke (1972) found reductions in noise level of 5-10dB for belts of trees between 15-30m wide. Research also suggests that wide plantings (around 30 meters) of tall dense trees combined with soft ground surfaces can reduce apparent loudness by at least 50%.	10 - 12m is optimum height	15 - 30m depth vegetation appears to have notable noise reduction benefit

On the basis of the findings from the literature review, the conclusions in Table 8‑2 were drawn regarding the threshold physical characteristics for vegetation to provide a noise reduction benefit.

Table 8‑2: Threshold physical vegetation parameters identified for providing noise reduction benefits

Parameter	Threshold value	Logic
Minimum vegetation depth	15m	Median average depth threshold for a noise abatement benefit identified in the literature review
Minimum vegetation area	200m2	Vegetation below this threshold area is assumed to provide 'little or no noise mitigation service' according to the ONS urban natural capital accounts (Office for National Statistics, 2019)
Minimum vegetation height	2.5m (and trees only)	Most evidence regarding vegetation height in the literature review pointed to the fact that only trees have tangible noise reduction benefit, hence only tree containing habitats should be considered. One study identified a stem height specifically of 2.5m.

To identify parcels of qualifying vegetation (which meet the above physical criteria) within the proposed scheme, Normalised Difference Vegetation Index (NDVI) imagery was used. This is a geospatial analysis that makes use of multispectral aerial imagery to assess how much light at certain wavelengths is being reflected by the surface at a given location. Healthy vegetation reflects more Near-Infrared (NIR) and green light compared to other surfaces, conversely it absorbs more light from red and blue wavelengths. Therefore, a calculation can be run on the NIR and red wavelength values for a given area to produce an NDVI value with a range of –1 to 1, indicating the presence of vegetation. A value closer to 1 indicates a higher vegetation density whilst a value closer to -1 indicates an absence of vegetation (e.g. hardstanding).

The other important element to understanding noise reduction is the number and type of receptors receiving this benefit. The available data on valuing noise mitigation benefits pertains to residential properties only; therefore, the OS Mastermap Buildings dataset was used, and buildings with a footprint greater than 15m² were removed as these are not likely to be residential. Research undertaken by Eftec (Defra, 2018) to develop the methodology for the UK NCA recommends that noise reduction benefits are valued only for buildings that sit within noise bands of 60dBA and above to avoid overestimating the benefits. Scotland’s Noise Map (Scottish Government and Transport Scotland, 2024b) was used to overlay noise bands with the receptor data to identify those buildings subject to noise above this threshold with a qualifying vegetation parcel (in the line of sight) between the building and the respective noise source.

A value of £120 per annum (2023 prices) from the ONS urban natural capital accounts (Office for National Statistics, 2019) was applied to qualifying buildings to calculate the baseline noise reduction benefit from vegetation. This value represents the lower-bound value of the health benefits to individuals living in UK buildings (within noise bands above 60 dBA) benefiting from road noise mitigation of at least 1 decibel by vegetation.

To understand any changes to baseline noise reduction benefits resulting from the proposed scheme, the impacts to any qualifying vegetation parcels were reviewed in terms of potential habitat loss. The post-development habitat map was overlayed to understand whether the proposed scheme would cause any qualifying vegetation parcels to drop under the threshold area (200m2) or depth (15m); in which case, the capacity for the parcel to provide a noise reduction benefit would be assumed to be lost. Vice versa, the post-development habitat map was also reviewed for any new vegetation (woodland – assuming only woodland would be of sufficient height) parcels which would meet the depth and area criteria and could be assumed to provide a noise reduction benefit based on their location relative to a noise receptor and source, as above.

 The assessment revealed a single, qualifying building within the proposed scheme under the baseline. Furthermore, the post-development habitat map suggested that whilst the relevant vegetation parcel was to be somewhat reduced in size as a result of the proposed scheme, the changes would not cause it to drop below any of the threshold physical characteristics for providing a noise reduction benefit. The qualifying building and the respective vegetation parcel shown under the baseline and post-development habitat maps are presented in Figure 8‑1. Additionally, no new qualifying vegetation parcels and buildings were identified as a result of the proposed scheme habitat creation. As such, the proposed scheme is not anticipated to have a tangible impact on noise reduction benefits and thus noise reduction was scoped out of the NCA.