This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

Economic unit values of soundscape/acoustic effects have been based on changes in the number of annoyed persons or on decibel changes. The normal procedure has been the application of these unit values to noise-attenuation measures affecting the noisier façade of a dwelling. Novel modular vegetation-based soundscape measures, so-called green walls, might be relevant for both noisy and quieter areas. Moreover, their benefits will comprise noise attenuation as well as non-acoustic amenity effects. One challenge is to integrate the results of some decades of non-acoustic research on the amenity value of urban greenery into design of the urban sound environment, and incorporate these non-acoustic properties in the overall economic assessment of noise control and overall sound environment improvement measures. Monetised unit values for green walls have been included in two alternative cases, or demonstration projects, of covering the entrances to blocks of flats with a green wall. Since these measures improve the noise environment on the quiet side of the dwellings and courtyards, not the most exposed façade, adjustment factors to the nominal quiet side decibel reductions to arrive at an estimate of the equivalent overall acoustic improvement have been applied. A cost-benefit analysis of the green wall case indicates that this measure is economically promising, when valuing the noise attenuation in the quieter area and adding the amenity/aesthetic value of the green wall.

Recent efforts of researchers, planners, architects, entrepreneurs and authorities dealing with increasing noise problems in European cities and urban areas have put an increased emphasis on the quality of soundscape [

Economic unit values of soundscape/acoustic effects have been based on changes in the number of annoyed persons or on decibel changes [

Green walls yield additional aesthetic/amenity qualities compared to traditional noise-attenuation measures, like noise barriers of wood or glass. The monetary value of noise and aesthetic/amenity qualities have been estimated in separate fields of research, although in both cases the value estimates have been based on hedonic pricing of dwellings, showing the percentage effect of e.g. urban greenery on sale prices [

The current economic valuation approach to soundscape/noise changes is either a valuation per person change from annoyed to not annoyed (or vice versa) or per dB(A) change per person (or household/dwelling) per year [

An overview of the benefits of adopting a broader approach including contextual factors is provided by Lercher [_{den} value of the most exposed façade should be taken into account. A challenge when valuing an improved soundscape at lower noise levels is that current approaches employ a high cut-off value. Improvements below the cut-off value are given zero economic value. As an example, all noise reductions below 50 dB(A) are regarded as providing no economic value [

Generally, in terms of acoustic benefit, vegetation affects the sound field in urban environments through three mechanisms: sound absorption and sound diffusion, which occur when a sound wave impinges on the vegetation and is then reflected back; and sound level reduction, when a sound wave is transmitted through the vegetation [

The reviewed green roof/wall valuation studies are based on the use of the hedonic pricing method, where it is estimated how much dwelling characteristics impact on the property price or the rent [

Peck

Gao and Asami applied a particular kind of hedonic pricing of greenery, ^{2} as a green wall quantity approximation to (each of) the two quality levels (25 m^{2} greenery for level 0 and an additional 25 m^{2} greenery for level +1), the resulted unit value estimates is of, respectively, ca EUR 3.5 per square metre green wall in Tokyo and ca EUR 1 in Kitakyushu. Obviously, these estimates are very sensitive with respect to the area assumption.

Ichihara and Cohen found that homes in New York with green roofs had as much as 16.2% higher price than homes without green roofs [

Value estimates, per household per year, green roof/wall ^{†}.

Study | Year Data | Country Data | Curr-ency | Property Price | Annu-ity | Gree-nery % Value | Gree-nery Value | EUR/ Curr-ency | CPI | EUR-2010 Gree-nery Value | Gree-nery Size (m^{2}) |
Unit Value (per m^{2}) |
---|---|---|---|---|---|---|---|---|---|---|---|---|

Peck |
1999 | Canada, Toronto | CAD | 230,000 | 12,599 | 10.50% | 1 323 | 0.6313 | 1.25 | 1,044 | 50 | 20.88 |

Hunt (2008) [ |
1999 | Canada, Toronto | CAD | 230,000 | 12,599 | 9% | 1 134 | 0.6313 | 1.25 | 895 | 50 | 17.9 |

Gao & Asami (2007) [ |
1999 | Japan, Tokyo | JPY | 602,400 | 32,998 | 1.40% | 8 400 | 0.0082 | 1.25 | 87 | 25 | 3.46 |

2003 | Japan, Kitakyushu | JPY | 73,200 | 4,010 | 2.70% | 1 980 | 0.0076 | 1.15 | 17 | 25 | 0.69 | |

Ichihara |
2000 | US, New York | USD | 73,024 | 4,000 | 16.20% | 648 | 1.0827 | 1.22 | 859 | 50 | 17.18 |

Des Rosiers |
1999 | Canada, Québec | CAD | 112,000 | 6,135 | 3.90% | 239 | 0.6313 | 1.25 | 189 | 50 | 3.78 |

Tomalty & Komorowski (2010) [ |
2010 | Canada, Toronto | CAD | 395,460 | 21,662 | 20% | 4 332 | 0.7325 | 1 | 3,174 | 50 | 63.47 |

2010 | Canada, Toronto | CAD | 395,460 | 21,662 | 7% | 1 516 | 0.7325 | 1 | 1,111 | 50 | 22.22 |

^{†} Values in currency of a study are converted to Euro in the year of the study, and these Euro values are adjusted to Euro 2010 values applying the consumer price index (CPI) for the Euro zone. A discount rate of 5% and a 50 year lifetime to calculate annuity values from the property values have been applied. If the study reports annual rental values, these are put under the annuity column. The “value” is the total green roof/wall annual value for the household. Since Hunt [

The mean of these value estimates is EUR 18.7 per square metre greenery per household per year, but the mean drops to EUR 12.3 if the highest estimate from Tomalty and Komorowski is removed. This would yield approximately 5.1 EUR per affected person (assuming a household size of approximately 2.4 persons). A 90% confidence interval from the household unit value distribution is from about EUR 3 to about EUR 60. Due to the relatively low number of observations these estimated average unit values are statistically uncertain and unstable. The estimates also depend on the assumptions regarding the size of the valued object, as well as other quality aspects that has not been possible to control for [

This paper assesses cases where vegetation-based noise attenuation measures affect the quieter façades of dwellings and the common residential areas, the inner courtyards. The estimated Miedema curves for annoyance have 37 dB(A) and 42 dB(A) as departure values for the proportion of people who report they are annoyed or highly annoyed respectively [

Meta-analyses are often used for synthesising results from various studies [_{g}_{g}_{g}_{g}_{g}^{2}_{g}

Some referred studies have not reported measures of uncertainty in the price effect of urban greenery. Some do report price effect intervals, while others report the standard error of the price effect parameter in the hedonic regression model. In the latter case an underlying normal distribution of the parameter and estimate variance as the square of the estimated standard error have been assumed. When upper and lower levels are reported, it is assumed that these represent the upper and lower values of 95% confidence intervals, and, by assuming an underlying normal distribution, the implicit standard error from dividing by 1.96 on each side of the mean is extracted. When measures of uncertainty are missing, it is assumed that the standard error is 50% of the mean estimate. Moreover, it is assume that these uncertainty estimates can be applied to the EUR-2010 unit value point estimate.

To get some idea on how the estimated unit values of urban greenery might impact on estimated benefits of noise-control measures with aesthetical/ambient qualities, a test applied to two green wall applications has been included. These are based on two configurations, typical for European urban settings. They consist of a block of flats, six storeys high, with eight apartments on each floor. Both configurations should be compared to reference configurations without a green wall. The reference configurations present a part of an urban setting that consists of an urban street canyon with cross streets and a roadside courtyard. The main street canyon dimensions are 48 m × 19.2 m × 19.2 m (length × width × height), the cross street dimensions are 66.8 m × 9.6 m × 19.2 m and the courtyards dimensions 19.2 m × 19.2 m × 19.2 m. Due to periodicity in the computational model, the main street canyon is infinitely long, ^{2} (^{2} (

All 48 apartments face both sides of the building, so 18 (3 × 6) apartments face a busy street as well as the courtyard, while 18 (3 × 6) apartments face the opposite side of the building and the courtyard, as can be seen from

(^{2}, on a block of flats, six storeys high, with eight apartments on each floor; (^{2} facade, on a block of flats, six storeys high, with eight apartments on each floor (Source: [

Top view of the one of the periods of the model representing the layout of the apartments at each floor.

With the assumed road traffic as sources of sound, the sound pressure levels are computed by a hybrid approach. For the low frequency range,

To illustrate the potential advantage of including the benefits from the aesthetic/amenity appreciation of the vegetated façades, we have undertaken simplified cost-benefit analyses of the two demonstration projects. The discount factor is set to 3% and the project horizon to 40 years [

The benefits of the green walls comprise the noise attenuation for quieter façades/inner courtyards and other non-acoustic amenity benefits, e.g., the aesthetic effect. All the 48 apartments have one (quiet) side facing the inner courtyard, and will be affected by the noise attenuation due to the green wall at the entrance façade. It is assumed that noise impacts are omitted from the reviewed amenity valuation of green roofs/walls, such that we can simply add the aesthetic/amenity valuation to the dB-change valuation. At the outset it is assumed that the weighted averages of square metre non-acoustic unit values of green walls from the literature survey are applicable to the case project buildings considered in this paper. All apartments also benefit from the improved quality of the building block as such, having common access to the courtyard with improved acoustic and non-acoustic (visual) quality, as well as when passing into and out of the building complex. In addition, several of the dwellings will have the greenery of the façade in the direct view from inside the apartments. In the described demonstration studies not only the sound quality of the quiet side of the façades is improved, but also important parts of the neighbourhood soundscape such as in the courtyard of the building. The perceived benefit of this neighbourhood improvement will depend on other non-acoustic quality of the actual courtyards [

Regarding project costs, these are additional costs compared to an alternative wall without greenery. The size of the façade improvements are of 58 m^{2} for the 3 m vegetated façade and 369 m^{2} for the 19.2 m façade. The wall is 9.6 m deep and the façades of both sides of the openings are covered. The cost of the vegetated façades is set to 500 €/m^{2}, with a lifetime of 10 years. A lifetime of 50 (40–60) years is probably an appropriate estimate for (modern) green roofs [^{2}. These values are applicable for the project year 2011. This yields an annual cost (in 2011) of 56.91 € per m^{2} (including the investment cost and the annual maintenance).

Since there are substantial uncertainties associated with all of the estimated costs and benefits, a Monte Carlo simulation has been undertaken as a sensitivity analysis of the impact from varying the benefits of the aesthetic appreciation with ±50% and all other benefits and costs with ±30%. It is assumed that the number of beneficiaries vary ±15%. The uncertainties are input to the Monte Carlo simulations by drawing from a truncated normal distribution with standard deviation equal to the uncertainty multiplied with the estimate itself. For an effect of 4.5 dB(A) the bulk of the values lying between ±30% that is ±1.35 dB(A) has been assumed that results in a standard deviation of 1.35. The truncation of the normal distribution has no substantial impact, and is simply used as a mechanism to ensure that negative values or values very close to zero are not obtained.

Weighting of unit values, green roof/wall ^{†}.

Study | Country Data | Uncertainty Measure | EUR-2010 Greenery Value | “Effect”, Unit Value (per m^{2}) |
W_{g} = 1/V_{g} |
---|---|---|---|---|---|

Peck |
Canada, Toronto | interval | 1,044 | 20.88 | 0.048 |

Hunt (2008) [ |
Canada, Toronto | interval | 895 | 17.90 | 0.027 |

Gao & Asami (2007) [ |
Japan, Tokyo | SE of parameter | 87 | 3.46 | 0.368 |

Japan, Kitakyushu | SE of parameter | 17 | 0.69 | 6.351 | |

Ichihara |
US, New York | SE of parameter | 859 | 17.18 | 0.014 |

Des Rosiers |
Canada, Québec | SE of parameter | 189 | 3.78 | 0.281 |

Tomalty & Komorowski (2010) [ |
Canada, Toronto | none | 3,174 | 63.47 | 0.001 |

Canada, Toronto | none | 1,111 | 22.22 | 0.008 |

^{†} 1/_{g}_{g}_{g}_{g}

Since the noise attenuation affects the quiet side, a scaling factor of 0.3 to adjust the benefits downwards has been used. Thus, from the nominal improvement of 4.5 dB(A) and 4.1 dB(A), the monetised noise valuation applies to, respectively 1.5 dB(A) and 1.37 dB(A). As a point of departure the HEATCO valuation of noise annoyance from 2002 have been used [^{2} green wall (

The benefit-cost ratios obtained from the calculations show that the sum of benefits is nearly four times as high as the costs. Also noteworthy is how amenity/aesthetic benefits are about 10 and 100 times higher than the benefits from the acoustic improvements. Given the tentative nature of the valuations and their application, the CBA for the situation where the low outlier value from the literature survey dominates the green wall unit value estimate, yielding 0.5 EUR per person (

The lower green wall unit value assumption yields benefit-cost ratios just above unity. Moreover, the efficiency ranking has been also switched, with the low green wall ranked first, as the acoustic performance plays a relatively larger role and the lower wall (lower entrance opening) yields best noise attenuation.

Calculation of benefit-cost (B/C) ratios, for the two demonstration projects with differently sized green walls—3 m high and 19.2 m high, assuming a green wall unit price of 2.4 EUR (2010) per m^{2} per person per year.

Measure | Size | Benefits | Costs | BC ratio |
---|---|---|---|---|

Green wall, 3 m façade opening | 58 m^{2} |
|||

Investment costs | 3,301 | |||

Maintenance costs | 1,450 | |||

Noise attenuation benefits | 1,932 | |||

Amenity/aesthetic benefits | 16,935 | |||

Total | 18,867 | 4,751 | 3.97 | |

Green wall, 19.2 m façade opening | 369 m^{2} |
|||

Investment costs | 20,999 | |||

Maintenance costs | 9,225 | |||

Noise attenuation benefits | 1,761 | |||

Amenity/aesthetic benefits | 118,698 | |||

Total | 120,458 | 30,224 | 3.99 |

Calculation of benefit-cost (B/C) ratios, for the two demonstration projects with differently sized green walls—3 m high and 19.2 m high, assuming a green wall unit price of 0.5 EUR (2010) per m^{2} per person per year.

Measure | Size | Benefits | Costs | BC ratio |
---|---|---|---|---|

Green wall, 3 m façade opening | 58 m^{2} |
|||

Investment costs | 3,301 | |||

Maintenance costs | 1,450 | |||

Noise attenuation benefits | 1,932 | |||

Amenity/aesthetic benefits | 4,234 | |||

Total | 6,166 | 4,751 | 1.30 | |

Green wall, 19.2 m façade opening | 369 m^{2} |
|||

Investment costs | 20,999 | |||

Maintenance costs | 9,225 | |||

Noise attenuation benefits | 1,761 | |||

Amenity/aesthetic benefits | 29,674 | |||

Total | 31,435 | 30,224 | 1.04 |

The sensitivity analysis of the CBA was accomplished by use of the Monte Carlo simulation (

Monte Carlo simulations of benefit-cost (B/C) ratios, for the two demonstration projects with differently sized green walls—3 m high and 19.2 m high, assuming a green wall unit price of 2.4 EUR (2010) per m^{2} per person per year.

Monte Carlo simulations of benefit-cost (B/C) ratios, for the two demonstration projects with differently sized green walls—3 m high and 19.2 m high, assuming a green wall unit price of 0.5 EUR (2010) per m^{2} per person per year.

As expected, for the lower unit price estimate for aesthetic/amenity values, the simulated benefit-cost ratio for the green wall installations is considerably lower, around 1, indicating that economic efficiency will be questionable (

The methodology and practice for economic assessment of noise-attenuation measures in quiet areas and vegetation-based noise attenuation measures have been incomplete. Valuation of noise attenuation has only been related to effects on the noisier façade of dwellings, not the effects on quiet areas, although soundscape improvements in quiet areas might be valued irrespective of an unchanged situation at the noisier façade [

Regarding the valuation of noise attenuation at the quiet façade, a slightly higher conversion factor than proposed by the CityHush project [

There are basic methodological challenges in the application of hedonic pricing, as in other methods for valuation of environmental goods, e.g., that attributes like greenery and/or noise might be overestimated due to omission of other. A crude unit value estimates for urban greenery, namely square metre values for green roofs/walls has been proposed. These unit value estimates are simply based on averages of valuation estimates from the literature, predominantly hedonic pricing. The valuation studies have been carried out in various countries in the OECD area, with a majority having US origin. These studies covered different scopes of green roof/wall projects. In this paper, a simple transfer to Euro and an update to 2010-prices using standard CPI have been only adjustments of the original values. No additional adjustments or weightings using purchase price parities or the scope of the original valuation have been used. The weighted mean estimate from the surveys was 1.2 EUR per square metre per household per year, which increased to 5.8 EUR when the lowest estimate that had a 90% impact on the weighted mean has been eliminated. While the first of this estimate would yield benefit-cost ratios around unity for the two green wall demonstration projects that have been assessed, the second estimate would yield benefit-cost ratios around four. The relative economic value weight from aesthetics/amenities is in any case considerable, compared to noise attenuation values, but the exact unit value will of course affect economic efficiency estimates for a given project.

This study represents only a first step in economic assessment of vegetation-based noise attenuation measures, including a unit value for non-acoustic amenity benefits. The proposed simplistic approach can be regarded as applicable for a pilot CBA or a “mini-CBA” [

This study was part of the project “Holistic and sustainable abatement of noise by optimized combinations of natural and artificial means” (HOSANNA), funded by the European Commission under the Sustainable Surface Transport of the 7th Research Framework Programme, Grant Agreement No. 234306. We are indebted to two anonymous reviewers to this journal. All remaining errors and omissions are entirely our own responsibility.