Green Infrastructure Evidence Base

4 Community Liveability


4 Community liveability

Green Infrastructure

Green Infrastructure is the network of green spaces and water systems that delivers multiple environmental, social and economic values and services to urban communities. This network includes parks and reserves, backyards and gardens, waterways and wetlands, streets and transport corridors, pathways and greenways, farms and orchards, squares and plazas, roof gardens and living walls, sports fields and cemeteries. Green Infrastructure secures the health, liveability and sustainability of urban environments. It strengthens the resilience of towns and cities to respond to the major current and future challenges of growth, health, climate change and biodiversity loss, as well as water, energy and food security.


4.1 Introduction

Green Infrastructure can enhance the general attractiveness and ‘liveability’ of urban neighbourhoods, and cities. The term ‘community liveability’ covers a wide range of factors. Some of these are ‘intangible’ benefits which are difficult to quantify, such as cultural or visual and aesthetic values. Other benefits such as human comfort are more easily quantified. The topic has not been as extensively researched as the other more easily quantifiable benefits of Green Infrastructure, and often involves qualitative rather than quantitative research methods. Much of the literature also relates to urban trees which, due to their scale, comprise the most visible component of Green Infrastructure in cities. Figure 19 summarizes the role ofGreen Infrastructure in enhancing community liveability.

Figure 19: Summary of the role of Green Infrastructure in community liveability (Source; M. Ely).

4.2 Overview

4.2.1 Definitions of liveability

Liveability is a familiar concept which addresses many aspects of urban life. A very broad range of factors contribute to making a place liveable so there is no single definition of liveability.In addition, the factors that contribute to liveability also vary for individuals depending upon their circumstances and life-cycle stage. The Victorian Competition and Efficiency Commission (the Commission) has developed a working definition for liveability (Victorian Competition and Efficiency Commission, 2008).

‘Liveability reflects the wellbeing of a community and represents the many characteristics that make a location a place where people want to live’.

This definition encompasses a wide range of characteristics of a liveable place including: community strength; economic strength; built infrastructure; social infrastructure; amenity and place; environment; citizenship; equity and human rights; participation; leadership and good governance; information and communication technology (ICT); transport; government services; and innovation.

Green Infrastructure can help to improve the liveability and quality of life in cities and urban neighbourhoods. In addition to the environmental and economic values described elsewhere in this report, the ecosystem services provided by vegetation, water and other Green Infrastructure provide a range of socio-cultural values that are important to humans because of social norms and cultural traditions. This set of related benefits can be grouped under a broad umbrella category of ‘community liveability’.

Many of the Green Infrastructure ecosystem services which contribute to community liveability are described elsewhere in this report. For example GI can enhance community liveability through

  • Promoting general human health and well-being (Chapter 3)
  • Facilitating community cohesion and social capital (Chapter 3)
  • Promoting the economic vitality of commercial centres (Chapter5)
  • Creating more amenable urban climates and microclimates (Chapter 6)
  • Integrating water into urban environments (Chapter7)
  • Enhancing biodiversity in urban areas (Chapter 8)
  • Production of  local fresh food (Chapter 9)

This section on community liveability focuses more specifically on the following categories:

  • Cultural values, including community heritage values and the deeper symbolic and other values of urban nature
  • The visual and aesthetic role of Green Infrastructure, including place making, spatial definition and contributions to the attractiveness of urban streets, neighbourhoods and city centres
  • Urban amenity, including the role of Green Infrastructure in creating more ‘walkable’ streets and more ‘liveable’ cities by enhancing human comfort, safety and enjoyment
  • The specific liveability benefits of air quality improvement and noise abatement in cities

4.3 Cultural values

4.3.1 Cultural meaning

Significant cultural meaning and symbolic value have become attached to trees and forests, including those of archetypes, myth and religion (Konijnendijk, 2008; Dwyer et al., 1994; Schama, 1995). (Dwyer et al., 1991) recommended adopting a broader perspective on urban trees:

‘Trees and forests play a significant role in theurban environment and have many importantmeanings to urban residents. However, we findthat the effort of many municipal urban forestryprograms to expand or sustain trees and forests isjustified in terms of a few fairly simple dimensionsof their significance to urbanites, such as beauty,shade, cooling, or contribution to global gas balances.Programs based on this narrow spectrumof tree benefits may not fully meet the needs ofurbanites or gain their support. We suggest abroader perspective is needed, one that takes intoconsideration the deep psychological ties betweenpeople and urban trees and forests’ p.276.

Research at the Morton Arboretum in Lisle, Illinois has identified a number of cultural themes including:

  • The sensory dimension of trees.The contribution of trees and forests to the beauty of the urban environment is well documented but their influence on people goes deeper than visual aesthetics. Trees and vegetation can have a strong, relaxing effect on people (Ulrich, 1981).
  • The symbolic value of trees. Apart from the sensory experiences they provide, trees are often valued as carriers of symbolic meaning. There are many examples of trees used as symbols of people, as well as in religions.
    • The symbolic value of trees, as symbols of people.(Appleyard, 1980) observes several parallels between our images of people and trees.
    • The symbolic value of trees as religious symbols. (Chenowth and Gobster, 1990) suggest that urban trees and forests can contribute to experiences that are religious in nature.(Schroeder, 1988) observes that trees have been used by many cultures to symbolize health, wisdom, and enlightenment.
  • Human roots in the forest. People's responses to trees and forests are so strong and consistent that some researchers have even suggested that people have evolved instinctive preferences for certain types of treed environments(Appleton, 1975; Appleton, 1984). Most people seem to appear to prefer groves of widely scattered trees, open at eye level, with overhead canopy and a uniformly textured ground cover. It has been suggested that this environment is attractive because it resembles the African savannah in which the human species evolved.

(Dwyer et al., 1991)also suggests a number of emotional ties to the actual act of tree planting including:

  • Tree planting as a demonstration of commitment to the future.
  • Tree planting as possibly a major impact on the landscape over time. Few activities that individuals can undertake have the potential for as large an impact over time as tree planting.
  • Tree planting as a means of improving the environment. Tree planting is one actan individual can perform thatcan help them feel they have contributed to solving global environmental problems.

Figure 20: Neighbourhood shadeways tree planting day, Brisbane City Council. Source: Lyndal Plant BCC.

Because of their longevity, tree planting also creates a legacy for future generations, and a link between generations (Moore, 2000b).

4.3.2 Heritage values

Trees also have cultural and heritage values, as in the creation of ‘avenues of honour’ (Cockerell, 2008). For example, "canoe trees" have indigenous heritage significance and the old gum tree at Glenelg in South Australia is significant as a symbol of colonial settlement.

4.3.3 Community values

Communities may place high values on trees compared with other aspects of their urban surroundings, and develop significant attachments to local trees (Hull, 1992). This is also reflected in reported preferences by residents for tree lined streets (Getz et al., 1982).

4.3.4 Place making

Historically trees have played a significant ‘place making’ role in cities, defining a ‘sense of place’ when configured as an individual shade tree, avenue or grove(Moore et al., 1988). Trees still continue to play a significant place making role in modern cities (Alexander, 1977; Arnold, 1980).

4.4 Visual and aesthetic values

4.4.1 Introduction

The visual and aesthetic benefits of trees and other forms of Green Infrastructure have received less attention in recent research, compared with the significant body of research on the environmental, economic and human health and well-being benefits of urban greening. In part this is because most research into Green Infrastructure has been quantitative in nature, and has been undertaken within the fields of environmental science, social science and economic modelling (McLean et al., 2007).Much less research has been undertaken by those well-versed inaesthetics and visual design principles (such as landscape architects and urban designers) and few studies have adopted a more appropriate qualitative or mixed-method research methodology.

4.4.2 Environmental quality and aesthetics

The visual appearance and attractiveness of towns and cities have been found to be strongly influenced by the provision of green space (Tibbatts, 2002).Environmental quality has two main components, the actual ‘physical’, and the more subjective ‘perceived’ quality of the local environment (Khattab, 1993). The concept of environmental quality is broad and can encompass many elements including environmental pollution and cleanliness, and visual quality and personal security.

4.4.3 The role of trees

A number of landscape architecture and urban design texts refer to the role of trees in the design of urban spaces, including streets:

  • Because of their size and longevity trees have long been a major element in urban landscape design (Dwyer et al., 1994).
  • Trees provide structure, connection, presence and scale, amelioration of harsh environments, and a capacity to link diverse landscapes (Moore, 2000b).
  • In her seminal 1961 book, The Death and Life of Great American Cities, Jane Jacobs noted the important role of street trees in creating visual unity in the modern city streetscape (Jacobs, 1961).
  • (Alexander, 1977) has noted the ‘place making’ role of trees in urban landscapes, either as a single tree, a grove of trees or a linear avenue (Alexander, 1977).
  • (Arnold, 1980) examined the role of trees in urban design in some detail, including street trees (Arnold, 1980).
  • (O’Brien, 1993) examined the aesthetic and planning contributions of street trees, comparing the cities of Birmingham and Munich (O'Brien, 1993).
  • Based on a number of international case studies, Alan Jacobs has examined the qualities that contribute to what are recognized as the worlds ’great streets’ and to multi-lane‘boulevards’ (Jacobs, 1993; Jacobs et al., 2002).

4.4.4 The role of large trees

Larger trees have greater visual presence than smaller stature trees, and are often more highly valued by residents, especially where ‘canopy closure’ over the street is achieved (Kalmbach and Kielbaso, 1979; Schroeder and Cannon, 1983; Sommer et al., 1989). In one study the single largest factor in determining the attractiveness of a street scene was the size of the trees and their canopies (Schroeder and Ruffolo, 1996). This was supported by a study in which there was a preference for large canopied trees in a tree replacement program (Heimlich et al., 2008). According to (Schroeder et al., 2009) ‘big trees’ have long been a significant feature in many cities and towns. A canopy of mature trees arching over the street and shading properties has defined the character of many urban and suburban communities. In fact it is the enduring nature of large trees in a rapidly changing urban environment that contributes to their high symbolic value and a sense of permanence in a fast changing society (Dwyer et al., 2003).

4.4.5 Local research

In 2009, Ely administered a national online survey to obtain a ‘snapshot’ of the attitudes and practices of street tree practitioners throughout Australia (Ely, 2010). Responses were obtained from 282 ‘street tree practitioners’ working in tree related fields, mainly in local government. Surprisingly the less tangible visual and aesthetic benefits of trees were rated highest, despite recent emphasis on the quantifiable environmental benefits of trees.

Figure 21: Perceived benefits of street trees. Source (Ely, 2010).

Similar annual surveys of residents by the Brisbane City Council also highlight the importance of aesthetics as a reason for tree planting. In October 2010, the most frequently cited benefits of planting trees were shade (59%), aesthetic appeal (49%) and environmental benefits (45%).Mentions of shade, the aesthetic appeal and habitat for wildlife increased significantly from October 2009. Since October 2008, the perceived benefits of clean air and greenhouse gas absorption from planting trees have been on the decline.

Figure 22: Perceived benefits of planting trees. (Source Brisbane City Council).

In a follow up study, Ely conducted in-depth interviews with street tree managers and related professionals from nineteen local government authorities in the Adelaide metropolitan region (Ely, 2009a). Structured in-depthinterviews were conducted in the respondent’s workplace and the interview transcripts were analysed using qualitative methods to identify emerging themes. Interviewees were asked to identify the main benefits of street trees. There was strong recognition of the visual and aesthetic role of trees in urban neighbourhoods, and the following categories were emphasized:

  • Urban amenity
  • Visual character
  • Streetscape appeal
  • Suburb desirability
  • Resident support for trees
  • Identity and legibility

Verbatim comments also emphasized the many visual and aesthetic values of street trees:

‘The usual list of environmental benefits is ok-but more often than not you notice when you drive down a street that has been upgraded. If you get good street trees you get good aesthetic appeal’.

‘Tree planting is a relatively cheap way of improving the amenity and character of an area. Trees provide shade. They soften the line of the road and infrastructure, and create light and shade’.

The economic, environmental and human benefits of street trees are well recognised by tree managers, however the benefit most emphasised is their mainly visual role in creating character and amenity in urban streets and suburbs. Street trees provide amenity, visual character and streetscape appeal. In established urban areas, the presence of mature street trees makes certain streets andsuburbs more desirable places to live, and their benefits are often reflected in higher real estatevalues. In such areas there may be strong local resident support for retaining existing trees.

‘When a person drives down the street, the biggest impact is the tree. And the leafiest suburbs, that’s why the value of the houses are higher than elsewhere, because it’s usually the leafy streets, or leafy suburbs’.

In developing outer suburban areas street tree planting also plays a significant role in creating localcharacter and identity. There was also an awareness of the relationship between urban greening and human well-being. However, this is less tangible and more difficult to communicate to the public than the more obvious visual benefits. In the words of one participant:

‘The most important benefit, they maintain quality of life and community. A lot of people don’t see that’.

(Marritz, 2012)supports the idea that the less easily quantifiable ‘visual and aesthetic’ benefits of trees are in fact the most powerful.

‘I read a lot of news about trees in the urban environment. These stories are usually about how important trees are, and frequently cite statistics like the number of tons of carbon storage and sequestration trees provide, the quantity of pollutants they remove, the amount of money they save us, or their contribution to controlling rising temperatures and flooding. I’m very familiar with statistics like these, because I also use them frequently. But are they really effective? Does data like this really matter to people?’

‘Trees do provide an incredible number of environmental and economic benefits. Despite this, it seems that it is often more persuasive to use visuals to communicate their value. Statistics can quickly become tiring and meaningless. Hearing them over and over again eventually dampens their impact. The average person won’t really take in what removing 711,000 metric tons of pollution feels like (that’s a $3.8 billion value, by the way). But we do know how nice it feels to walk on a tree-lined street. Especially in the hot new climate that is no longer an abstract future’.

(Ely, 2010) also interviewed street tree managers in the Australian capital cities of Sydney, Melbourne, Brisbane and Perth, to identify the strategic ‘drivers’ behind street tree planting in the most difficult urban environments. Despite local differences, each city has a common agenda in terms of creating urban amenity inthe Australian climate, and creating a high quality and inviting public realm. Each city sees trees as part of the image of its State’s civic, retail and tourism focus. The public realm of a major city mustmeet the needs of both residents and daily visitors (workers, shoppers and tourists). Street tree planting in capital city CBD’s is therefore considered to be ‘mandatory’, despite the many constraints on tree planting. These include narrow footpaths, high pedestrian volumes, overshadowing by tall buildings, extensive paved surfaces (often on a concrete slab base), extensive underground services established over the preceding century, and more recent drought and water restrictions. Specific agendas identified for Australian cities include:

  • Melbourne. Trees are part of a pedestrian friendly city centre. For the past 20 years, Council has been pursuing an innovative program to enhance pedestrian needs over vehicle needs in the central city. The process began in the mid-1980s with the urban design report Grids and Greenery (City of Melbourne, 1987). In the mid-1990s the Danish urban designer, Jan Gehl measured pedestrian conditions and activity on the ground. He looked at urban amenity from all perspectives, including the role of vegetation. The study was replicated in 2004, showing measurable improvements in public and street life following the implementation of recommended streetscape improvements (City of Melbourne, 2004).
  • Sydney. Trees are fundamental to city projects in providing civic amenity and upgrading the public domain in Australia’s largest CBD, despite the constraints imposed by its narrow street network lined with tall buildings.
  • Brisbane. Creating liveability and responding to the need for year-round shade and ‘shade-ways’ is a key policy ‘driver’ in Brisbane’s subtropical climate. Trees and greenery are also seen as creating liveability as part of the city’s urban densification program (Plant, 2006).
  • Perth. Trees are seen as a fundamental ‘given’ in a hot dry city and are an integral part of the ‘pedestrianization' of the city centre.

Figure 23: Swanston Walk Melbourne. By author.

4.5 Urban amenity

Amenity is a term referring to ‘the pleasantness or attractiveness of a place’ or to ‘the desirable or useful features or facility of a place’. Areas with high levels of amenity are more ‘pleasant’ or ‘attractive’ places to live, work or visit. The concept of urban amenity includes not only the visual and aesthetic qualities of a place, but also a range of more functional considerations such as safety, comfort and convenience. Green Infrastructure can contribute significantly to the amenity of urban streets, neighbourhoods and cities. Research has focused on a number areas:

  • Neighbourhood attractiveness
  • ‘People friendly’ streets and spaces
  • Walkability
  • Green Infrastructure and pedestrian/driver safety

4.5.1 Neighbourhood attractiveness

Increased greenery within urban areas (including street trees and landscaping, water bodies and access to open space) increases the aesthetic value of neighbourhoods. The positive impact of Green Infrastructure practices on aesthetics is reflected in the relationship between urban greening and property values. People appear to be willing to pay more to live in places with more greenery. Several empirical studies have shown that property values increase with the presence of trees and other greenery, and this is reinforced by research into the preferences of householders.

4.5.2 Improving quality of place

It has been shown that Green infrastructure and green space provision can make positive contributions to improving‘quality of place’ (Forest Research, 2010).‘Quality of place’ has been defined as the physical characteristics of a community that affect the quality of life and life chances of people living and working in it (Cabinet Office Strategy Unit, 2009). Research shows that that the provision of high quality, well-maintained green space can have a positive effect on local business, and improve an area’s image and the confidence of the local population and potential investors (Land Use Consultants, 2004). (Swanwick, 2009) has noted that highly valued green spaces enhance positive qualities of urban life, offer a variety of opportunities and physical settings and encourage sociability and cultural diversity. Another study showed that, conversely, poor quality green space can negatively affect local activities and business and undermine an area’s image and the confidence of the local population and potential investors (Land Use Consultants, 2004).

4.5.3 Principles for liveability

A new report from the (Urban Land Institute, 2013) using Singapore as a model, offers a top ten list of principles for making high-density cities more liveable. The report ‘10 Principles for Liveable High Density Cities: Lessons from Singapore’ draws upon Singapore’s successful urbanization experience. Despite its population density, the city-state has consistently ranked favourably in various surveys measuring the liveability and sustainability of cities around the world. The ten principles in the publication were developed during two workshops hosted in 2012 by Singapore’s Centre for Liveable Cities and Urban Land Institute Asia Pacific, bringing together 62 leaders, experts and practitioners from different disciplines related to urban planning and development. Each of the ten principles in the publication reflects Singapore’s integrated model of planning and development, which weaves together the physical, economic, social and environmental aspects of urban living. The ten principles are:

  1. Plan for long-term growth and renewal
  2. Embrace diversity, foster inclusiveness
  3. Draw nature closer to people
  4. Develop affordable, mixed-use neighbourhoods
  5. Make public spaces work harder
  6. Prioritise green transport and building options
  7. Relieve density with variety and add green boundaries
  8. Activate spaces for greater safety
  9. Promote innovative and non-conventional solutions
  10. Forge 3P (people, public, private) partnerships

Principle 3, (Draw nature closer to people) relates specifically to urban greening. According to the report (p.31):

‘Blending nature into the city helps soften the hard edges of a highly built-up cityscape and provides the residents with pockets of respite from the bustle of urban life.

What started as an aim to build Singapore into “a garden city” has now evolved into Singapore being “a city in a garden”. In addition to the many parks scattered across neighbourhoods, water bodies course through the city and form an important part of the landscape. Nearly half of Singapore is now under green cover, which is not only aesthetically pleasing, but also is good for the air quality and mitigates the harsh heat of the tropical sun.

Another aim of having Singapore residents experience nature as an integral part of their lives is to encourage them to value and, as a result, take better care of the environment and the city’s limited natural resources’.

In creating ‘a city within a garden’, Singapore, due to high density and land scarcity, faced a practical limit on how many parks it could provide. So instead of only making horizontal spaces greener, Singapore adopted a strategy of ‘pervasive greenery’, meaning the city inserted greenery wherever it could (be it a pavement, road median, building facade, a rooftop). The idea was to ‘cloak spaces with green wherever the eye could see’.

Singapore also has a Streetscape Greenery Master Plan. Tree-lined roads provide shade for motorists and pedestrians while overhead bridges and flyovers are veiled with creepers and other plants to create a softer feel to concrete structures. The city has also introduced various methods to bring greenery to buildings, for instance green roofs, rooftop gardens, greening of vertical walls, and landscaped balconies. The National Parks Board has actively promoted vertical greening through incentive schemes and awards, and has even published extensive guides on ‘skyrise greenery’. Singapore has managed to create many tiers of highly visible greeneryfrom ground level up to the building tops, seeing the high-rise structures that form the cityscape as a means, rather than an impediment, to introducing more greenery.

4.5.4 People friendly streets and spaces

Streets are probably the most important element of the public domain, and are fundamental to the human experience of the city. Traditional streets provided a balance between pedestrians and other uses. However in the twentieth century streets have focussed predominantly on facilitating vehicle movement and accommodating engineering infrastructure. A more balanced concept of street design recognizes the need for pedestrian amenity and public life, as well as fulfilling these engineering functions. Such streets are currently known by a number of names such as ‘walkable streets’, ‘complete streets’, ‘liveable streets’ or ‘context sensitive streets’.

Most recently environmental sustainability has been added as another ‘layer’ to the street design agenda. Streets have a significant potential to provide the city with a wide range of ecological services in the form of urban greening, climate change adaptation, urban heat island mitigation and sustainable stormwater management (WSUD), as well as encouraging ‘active transport’ modes which promote community health while minimizing greenhouse gas emissions.

Gehl Architects have developed a set of criteria to assess quality of the public domain,and when all of these criteria have been fulfilled it will result in a place where people ‘can use all the human senses and fully enjoy walking as well as staying’(Gehl, 2002; Gehl and Gemzoe, 2003; Gehl, 2008). These quality criteria are divided into three groups: protection, comfort and enjoyment:

  • Protection focuses on how to minimize unpleasant experiences including crime, traffic accidents and unpleasant climatic conditions.
  • Comfort deals with the quality of walking and staying in a place, and involves walking, standing and sitting as well as the possibility for seeing, hearing and talking.
  • Enjoyment covers the human scale, enjoying the positive aspects of the climate and the experience of the qualities of the design of the place, including quality of materials used.

Alan Jacobs has examined the qualities that contribute to streets which are universally recognised as ’great streets’ (Jacobs, 1993) and to multi-lane urban boulevards (Jacobs et al., 2002). Jacobs has concluded that street trees are more than just ‘aesthetic decoration’, and are in fact one of the most important elements in the design of the urban streetscape:

‘Given a limited budget, the most effective expenditure to improve a street would probably be on trees. Assuming that trees are appropriate in the first place, and that someone will take care of them, trees can transform a street more easily than any other physical improvement. Moreover, for many people trees are the most important single characteristic of a good street’.

(Jacobs, 1993) considers that street trees can make a significant contribution to each of the following qualities of a street:

  • Places for people to walk with some leisure. Trees planted along a kerb, especially if closely spaced, define a pedestrian zone separated from vehicular traffic, creating a sense of safety both physically and psychologically. Trees planted in the parking lane may also help to bring it visually into the pedestrian realm.
  • Physical comfort. Street trees provide physical comfort for pedestrians by providing shelter from wind and rain in winter and cooling shade in summer. Deciduous trees also provide access to warming sun in winter. Closely spaced trees can provide continuous shade and shelter in all seasons.
  • Definition. Street trees provide physical definition, vertically with ‘walls’ of tree trunks, and horizontally with ‘roofs’ of tree canopies. The most satisfactory proportions between street width and the height of its vertical edges have long been a concern of architects and city designers.
  • Qualities that engage the eye. Street trees add visual complexity to the streetscape, and have the special attribute of a constant sense of movement of leaves and branches, and changing patterns of shade and light. Green is also perceived as a restful colour in urban settings.
  • Transparency. Trees provide a semi-transparent edge to the street.
  • Scale. Trees can also provide a sense of human scale in the streetscape.
  • Design unity. Trees create a sense of design unity, especially with relatively close spacings and continuity of planting.
  • Maintenance. In the best streets, trees appear cared for, and adequate funds need to be allocated for their maintenance.

(O’Brien, 1993), in his book Street Trees for Cities and Towns, identifies a similar list of the functional and aesthetic contributions of trees to urban streetscapes. These include:

  • Creating or reinforcing identity in a street.
  • Complementing historic or culturally significant buildings or streetscapes.
  • Enhancing pedestrian or vehicular orientation, legibility and way finding. Street trees can emphasize direction and directional change by accentuating road lines. They can emphasize a sense of movement, and their spacing can be manipulated to create a desired ambience, with closer spacings emphasizing a sense of speed. Trees can also be used to emphasize road junctions and focal points, and to reinforce the hierarchy of streets within the city.
  • Playing a symbolic or monumental role, for example in major boulevards and city gateways.
  • Enhancing visual amenity through screening unsightly views, softening the mass of large buildings, and reducing the apparent width of streets (especially if trees are planted adjacent to the kerb).
  • Providing visual interest, colour and a sense of movement in urban settings.
  • Providing awareness of seasonal change.
  • Providing a sense of human scale by creating smaller spaces within the wider streetscape, both vertically (by creating ‘walls’ of tree trunks), and horizontally (by creating ‘roofs’ of tree canopies).
  • Providing clear spatial definition in streets, for example by separating pedestrian and vehicular zones, both physically and psychologically.
  • Most significantly, street trees can provide a unifying element in an often visually diverse and sometimes chaotic urban streetscape. This design unity, however, does not become ‘boring’, as it is coupled with a sense of variety, with each tree having its own individual character.

4.6 Walkability

Parks have long been seen as the most important sites for physical activity which can contribute to human health (Maller et al., 2002). More recently there has also been a focus on the role of streets as venues for walking to maintain health, especially for older people. Green elements, such as street trees, gardens and parks, have been found to be important aspects in making streets attractive for walking (Borst et al., 2008).Research in the last decade has broadened the range of public health issues associated with the design of roads and streets. One area of research has investigated the effects of different roadway designs and roadside designelements on ‘walkability’. Some researchers have attempted to objectively measure the impacts of street design on the walking behaviour of an area’s residents or workers. It has beenfound that the ‘walkability’ of a street or neighbourhood depends on a number of factors. For example (Southworth, 2005) reviewed a number of pedestrian plans and concluded that walkable neighbourhoods are associated with improved physical and mental health, and increased community vitality. Southworth identified the common characteristics of walkable neighbourhoods, which comprise:

  1. Connectivity
  2. Linkage with other modes
  3. Fine grained land use patterns
  4. Safety
  5. Quality of path (attention to details such as width, paving, landscaping and lighting)
  6. Path context (the path context is visually stimulating to the pedestrian)

(Zacharias, 2001) reviewed studies of pedestrian behaviour and highlighted the link between the ‘legibility’ or visual understanding of the pedestrian network and pedestrian travel on that network. His study suggested that both regular forms and spatial differentiation are important in enhancing walkability. He also identified the need for some complexity of space to maintain attention, but avoiding excessive complexity which may be considered dangerous. This reflects the work of  (Kevin, 1960) an early seminal figure in urban planning, whose study of pedestrians led to the concept that places should be ‘imageable’ (i.e. the pedestrian should be able to hold the image in their head).

With respect to the role of aesthetics in ‘walkability’ (Saelens and Handy, 2008) analysed 42 reviews of built environment correlates of walking. They found that six studies related aesthetic qualities and walking, however the measure of aesthetics varied between studies. (Naderi, 2003) examined five pedestrian pathways in Texas built in the clear zone of a roadway, to better understand the characteristics of pathways that attract walkers. It was found that walkers tended to associate the ambience or context of a place with its desirability for walking. In addition the edge of the space (defined as either a sense of enclosure, environmental identity or ‘genius loci’) was the single environmental variable that distinguished ‘good’ from ‘not good’ walking sites. It was also found that respondents preferred a ‘natural’ edge to an ‘urban’ edge, if walking specifically for health purposes. Naderi did not define a ‘natural’ edge but it was concluded that lining a path with trees (as opposed to having no barrier between the pedestrian and traffic) would contribute to a more ‘natural’ edge.

(Lee and Moulden, 2006) examined the differences between walking for recreation or for transportation purposes, to better understand how to promote both activities. Based on a survey of 438 participants, a model was developed to differentiate between the two activities. The data indicated that the environmental variables associated with walking for recreation differ from those associated with walking for transportation, (including the number of street trees present). Distance was much more important for walking for transportation, as was the presence of street trees. Environmental variables were more strongly associated with frequent rather than moderate walking, suggesting a ‘supportive physical environment’ may be a key element in promoting recommended levels of walking for health.

The perception of safety is an important component of walkability, and a number of authors have noted the possible pedestrian safety aspects of tree planting in the verge between the footpath and roadway. This includes both an increased perception of safety (by separating pedestrians and moving vehicles) and a real improvement in safety by creating a protective barrier which reduces the risk of being hit by a ‘run-off-the-road’ vehicle (Jacobs, 1993; CTRE, 2008).

4.7 Pedestrian safety

Statistics indicate that on average there are 11 pedestrian fatalities and 87 pedestrians seriously injured in metropolitan Adelaide annually.The majority (71%) of these serious casualty crashes occur where a pedestrian is struck at mid- block rather than intersection locations. Research into pedestrian safety has focused on the role of traffic calming measures, which can be similar to the desirable characteristics that make a street more ‘walkable’.

There seems to be a self-evident connection between pedestrian safety and traffic calming. Traffic calming has been shown to be particularly important where fewer pedestrians are present, as the presence of large numbers of pedestrians seems to encourage drivers to drive more slowly (Jacobsen, 2003). A number of researchers have sought to understand the impacts of different traffic calming measures. (Litman, 1999) reviewed over 100 research studies into traffic calming, and found that ‘comprehensive’ traffic calming measures increase safety both directly (by reducing vehicle speeds) and indirectly (by encouraging higher levels of pedestrian activity, leading to increased driver awareness). Reducing vehicle speeds is an important priority as there is an established correlation between higher vehicle speeds and both the occurrence and severity of pedestrian crashes. In 2003, the default urban speed limit in South Australia was reduced from 60km/h to 50km/h, and studies into the effects of this change in speed limit found that the number of hit-pedestrian casualty crashes significantly decreased by 21% in the first 3 years following the change(DPTI, 2011).

Research shows that traffic calming techniques (particularly speed humps) are effective in reducing driver speed, and importantly that a combination of techniques is more effective than installing a single technique (Daniel et al., 2005). It has been shown that traffic calming is best achieved with a combination of roadway (such as road humps) and roadside (such as trees) design elements. Research confirms that a number of such measures can contribute to pedestrian safety, but it is difficult to generalize from the research to quantify the specific impacts that a particular roadside element(such as trees planted in the verge) will have on pedestrian safety in a particular street. One study by (Topp, 1990) of nine urban streets in Germany found that a reduction in perceived roadway width led to reduced traffic speeds. Street trees were also identified as the most influential element in reducing perceived roadway width. Although there is little existing research on the effects of roadside planting on pedestrian safety, it is widely recognized that narrower roads appear to be associated with lower traffic speeds, and fewer pedestrian/vehicle accidents, and that street trees are one of the most effective means of reducing this perceived roadway width.

4.8 Driver safety

4.8.1 Introduction

One of the major challenges in the design of streets is the apparent conflict between driver safety and the benefits of providing roadside trees. In fact early traffic engineering publications often promoted the use of trees in enhancing roadway design. In 1949, Neale suggested that ‘trees have undoubtedly saved many lives and prevented many accidents in intangible ways’ and noted that well-spaced trees could improve driver comfort by providing protection from sun and wind, helping keep drivers alert, and reducing cross-glare (Neale, 1949).Others such as Ziegler have also noted the benefits of trees and landscaping in terms of providing shade, windbreaks, visual buffers and physical protection for pedestrians from run-off-the-road vehicles (Zeigler, 1986).

More recent transportation research and policy making, however, tends to connect roadside trees with an increased probability of road crashes, and an increased accident severity. In many areas traffic authorities have severely restricted roadside tree planting by enforcing ‘clear zones’ (areas of roadside are to be kept free of rigid objects such as trees above a specified trunk diameter). It is now recognised, however, that street trees can provide a wide range of benefits to street users (including enhanced comfort and walkability) which leads to increased levels of physical activity and psychological well-being. In additionthe recent focus on sustainability and climate change has emphasized the Green Infrastructure benefits of trees and roadside landscaping. It is evident that there is a need for a more balanced approach to street design, with consideration of social and environmental responsibility, as well as responsibility for user safety. Recent research has revisited the assumption that roadside trees should be eliminated for safety reasons.Research into the impacts of roadside design elements(such as trees) on driver safety tends to be grouped into two main categories (Macdonald and Supawanich, 2008).

a) Relationships between roadside design elements (such as trees) and driver speed and behaviour. (For example the influences of roadside landscaping on driver stress, and the idea of using trees as ‘environmental references’ to reinforce desired speed limits).
b) Associations between roadside design elements (such as trees) and the frequency and severity of vehicle accidents. (For example by correlating accident rates with the presence of roadside design elements such as trees).

4.8.2 Driver behaviour and roadside trees Vehicle speeds

It is well established that higher vehicle speeds are associated with more frequent and more severe crashes (Richter et al., 2006). As well as enforcing legal speed limits road authorities have also sometimes attempted to implement ‘environmental reference speeds’ using traffic calming devices, especially the visual narrowing of roadways. A number of studies support this concept of using roadside design elements such as trees to reinforce lower vehicle speeds. In Florida, (Dumbaugh, 2005) studied two sections of an arterial highway of the same width but with different cross-section designs. One section comprised ‘liveable’ street treatments (including wide footpaths, streetscape elements close to the edge of the roadway, a narrow clear-zone, and trees planted between the footpath and roadway). Over the five year study period, the section that was designed as ‘liveable’ was found to be safer ‘in all respects’ including fewer collisionsand fewer pedestrian and bicyclist injuries and fatalities. Dumbaugh hypothesized that people operate using ‘risk homeostasis’ (a subconscious evaluation of risk which guides behaviour). Where safety is ‘built-in’ by engineers, for example by providing wider traffic lanes, the human brain subconsciously interprets the increased provision of space as permission to be less careful and alert. But where the need for lower speed can be communicated through the physical design of the street the driver will tend to be more attentive and careful. Dumbaugh also speculated that this phenomenon may cancel out the desired safety effects of passive safety design measures adopted by traffic engineers.

Successfully planting trees in a narrow road corridor may involve reducing road laneway widths to create additional planting space.This is often assumed to reduce vehicle and bicycle safety. One study investigated the relationships between lane widths and safety on urban and suburban arterials in Minnesota and Michigan. The research found no indication that mid-block lanes narrower than 3.6 metres lead to increased crash frequencies (Potts et al., 2007).This suggests that strategies of narrowing traffic lanes to enhance pedestrian safety can possibly be implemented without impacting on driver safety. In another study, (Van der Horst and Ridder, 2007) conducted a laboratory simulation to investigate the influence of roadside infrastructure (including trees planted at different distances from the roadway) on driver speed and vehicle positioning (in this instance on a multi-lane highway). The findings appear to corroborate the idea of ‘environmental reference’, as drivers reduced speeds when encountering trees close to the roadway, suggesting that they ‘read’ their surroundings and react accordingly. Overall this research shows the importance of road design in communicating desired speeds to drivers, including the use of roadside features such as trees

Influencing driver behaviour by changing the visual characteristics of a road has come to be called the self-explaining-roads (SER) approach (Theeuwes and Godthelp, 1995). The SER approach, most well known in the Netherlands and the UK, and to some extent in New Zealand, focuses on three main principles of functionality, homogeneity and predictability, aimed at eliciting the appropriate driver behaviour for a range of specific road types. One aspect of SER is the development of perceptual cues in the roadside environment that suggest a particular speed or lane position. Consistent use of such cues on a hierarchy of road types allows drivers to correctly categorize a specific road type and automatically respond with the appropriate driving behaviour. The SER approach differs from the application of localized traffic calming measures, which rely on physical obstacles which are often unpopular with both drivers and residents, and lead to the re-emergence of problems at other locations. In New Zealand, (Charlton et al., 2010) developed a range of SER designs for different functional  road categories, using local design elements. Urban landscaping was used to create restricted forward visibility as a defining feature on the ‘local road’ category, with a design speed of 30 km/h. The increased landscaping consisted of trees planted in the centre of the road, and landscaped ‘community islands’ at periodic locations along the kerb sides. Speed data collected three months after implementation showed a significant speed reduction on local and collector roads. Driver stress reduction

Commuting has been shown to be one of the most stressful experiences of modern urban life (Novaco et al., 1990) and stress responses have been documented for all driving experiences, with intensity varying according to  traffic and road conditions (Rutley and Mace, 1972).

Researchers have suggested that roadside design can actually help to reduce driver stress. This builds on more general research showing that views of nature can have restorative effects and reduce stress (Ulrich et al., 1991; Kaplan, 1995a). (Kaplan, 1995a) hypothesized that natural settings can provide the variables needed to restore the mind from ‘directed attention fatigue’ which occurs as a result of over-concentration during tasks which do not seem to be physically demanding, such as driving. This fatigue is a key factor in human error, leading to a reduced ability to comprehensively survey, understand and respond to situations, and choose direction of course. This loss of attention is especially dangerous in tasks where there is a risk of harm, such as driving. Kaplan hypothesized that one way to restore attention is to ‘give the mind a break’ and let it focus on something that promotes involuntary attention, which he labelled ‘fascination’. Kaplan identified three elements necessary for ‘fascination’: the subject must shift attention to a separate environment, which must be rich and complex enough to create an alternative world, and this environment must be compatible with what the person would like to do. It is evident that natural settings often include all of these variables. This research points to the need for mind ‘fascinating’ roadside elements, especially natural elements. Kaplan’s research also indicates that directed attention fatigue can precipitate stress, and the presence of nature can potentially mitigate both conditions and make driving safer.

(Parsons et al., 1998) have examined the effects of a number of different driving environments on stress and found a connection between roadside greenery and stress recovery. 160 participants in a laboratory simulation were exposed to a ‘stressor’ then to either an urban or natural driving scene. The participants exhibited negative emotional responses and prolonged stress recovery periods after viewing predominantly ‘man-made’ driving scenes. In contrast, driving scenes through park like settings resulted in the least effect on blood pressure and the quickest recovery from stress. (Cackowski and Nasar, 2003) hypothesized that exposure to roadside vegetation may reduce the levels of anger and frustration often associated with driving. In a controlled laboratory experiment, they measured anger levels after subjecting 106 subjects to a stress inducing video. Subjects then watched one of three driving videos: a ‘built up’ highway with little vegetation, a ‘garden highway’ with some vegetation, and a ‘scenic highway’ with intensive vegetation. The researchers then measured anger levels, and frustration tolerance. Although the results did not find any effects on anger levels, they did suggest that roadside vegetation increases a driver’s levels of frustration tolerance. The findings also suggested that viewing natural driving scenes contributed to clearer thinking than viewing built-up environments.Although based on laboratory simulations  this research does suggest  that the presence of more natural roadside elements, such as landscaping, could play a role in reducing levels of driver fatigue, stress and frustration, which can impact on driving safety.

4.8.3 Tree crash studies

A number of researchers have compared accident statistics from similar road sections, but with different roadside treatments. It must be noted that some research into tree crashes does not adequately address the critical question of crash causation, particularly the influences of driver fatigue, speed or alcohol. For example if any of these factors are involved in a crash with a tree, is it appropriate to attribute the cause of the crash, or its severity, to the tree (leading to recommendations for tree removal),or to driver behaviour (leading to recommendations for improved behavioural control) (Macdonald and Supawanich, 2008)?

One study reviewed 1999 run-off-the-roadway collision data from fourteen US state transportation authorities and found that 8% of all fatal accidents involved trees, and that 90% of these accidents occurred on two-lane roads (Neuman et al., 2003). Only 24% of crashes with trees occurred in urban areas, and nearly half of those on curved roadway sections. While the study advocated the use of clear zone policies, it has been noted that it failed to consider the role of vehicle speeds in crashes which may have led to erroneous conclusions about crash causation.

(Mok and Landphair, 2003)compared accident rates on parallel ‘freeway’ and ‘parkway’ road sections in different parts of the US. Freeways were characterized by paved shoulders, concrete median barriers and wide vegetation clear zones. Parkways were characterized by grassed shoulders and medians, with roadside trees and other landscaping. Parkway sections exhibited lower fatal accident rates. Fatal accidents involving reflected glare and headlights only occurred on the freeway sections. No fatal collisions with trees were recorded in the survey period, despite the extensive level of landscaping. The difference between urban freeway and urban parkway sections was twice that between rural freeway and rural parkway sections, suggesting that landscaping urban corridors may have greater safety impacts than landscaping in rural corridors.

(Dumbaugh, 2006) conducted an empirical analysis of the safety of three roadside treatments in Florida: widened paved shoulders, widened fixed object offsets, and liveable street treatments (defined as including on-street parking, trees, footpaths and buildings located close to the roadway). Crash data were collected for 27 miles of the three different designs. After controlling for other variables, modelling indicated that only the liveable-street treatment was consistently associated with reductions in both roadside and mid-block crashes. To better understand these findings Dumbaugh also examined characteristics of the roadside crash locations for  tree and utility pole crashes. His analysis found that the probability of a tree or pole crash declined at greater than 5 metres offset. In addition 65-83% of urban fixed-object collisions did not result from the vehicle errantly leaving the roadway, as is often assumed, but from failing to properly negotiate a turning manoeuvre. Liveable street segments experienced 67% fewer roadside crashes than the other segments, suggesting visible fixed hazards induce behavioural changes which lead to slower driving speeds. Dumbaugh also undertook an informal study gauging the speeds of random cars approaching and passing through a liveable street intersection with 1.2 metre offset trees, which indicated a notable slowing of traffic, which suggests that drivers were adjusting their speeds in response to the surrounding environment.

(Lee and Mannering, 1999) related two years of accident data and roadside characteristics for a 60 mile length of highway in Washington State. Their analysis indicated differences in the safety characteristics of urban and rural highway sections. For urban highways, wide traffic lanes and wide shoulders were associated with a greater frequency of run-off-the-roadway accidents, but the presence of trees was negatively associated with such accidents. For rural highways, higher speed limits were associated with a greater frequency of run-off-the-roadway accidents.

A California study by (Sullivan and Daly, 2005) found that large trees in kerbed central medians in major urban and suburban highways were associated with an increased accident frequency,  and with more severe left-side collisions, although some associations  were not statistically conclusive. It is also possible that if some of these accidents had not involved a collision with a median tree, the vehicle could have crossed the median into on-coming traffic.  

A number of researchers have also conducted ‘before and after’ studies comparing accident statistics on stretches of roadway both before and after landscaping measures have been implemented. In a study of five urban arterial streets in Toronto, Canada, Naderi, (2003) found that landscape improvements reduced mid-block accident rates by 5-20% (for the  three years before and after the improvements) as well as increasing pedestrian use of urban arterials. Trees themselves could not be directly linked with the results. However,  it was observed that a well-defined road edge may lead to drivers being  more attentive and cautious. Using a ‘willingness to pay’ formula Naderi also found that the landscape improvements led to a saving in over CD$1 million in reduced traffic crashes within 3 years of implementation of the CD$2.5 million improvements. In Texas, (Mok et al., 2006) conducted a before and after study of ten urban arterials in which landscape improvements were implemented (including roadside and median greening, footpath widening and tree planting). The researchers compared accident statistics over a period of 3-5 years both before and after improvements, and found a decrease in crash rates after the improvements. This included decreased crash rates (from 1% to 71%) at eight out of ten sites. Overall tree collisions were reduced by approximately 71% for all road sections combined, and pedestrian fatalities by approximately 47%. The findings suggest that there may have been a driver response to the increased landscaping resulting in improved traffic safety.  It was noted that ‘the landscape not only contributes to greater aesthetic compatibility between the urban environment and the highway but may contribute to a safer street’.  In Washington State, a before and after study of landscaped medians and other streetscape improvements on two miles of an urban arterial road, concluded that tree variables had little impact on the prediction of crash rates (St. Martin et al., 2007). Data analysis revealed high crash rates before and after improvements, however shifts in crash locations did occur, from mid-block crashes to intersection crashes, due to the installation of medians, kerbs and landscape plantings. The researchers concluded that tree effects were probably masked by other variables. The study highlighted the difficulties of attributing causation in such ‘before and after’ studies, when a large number of changes are implemented.

It is also of interest to compare the differences between drivers’ perceptions of safety in tree lined streets and the assumptions underlying clear zone policies. (Naderi et al., 2008) studied how drivers' perceptions of highway safety were affected by the presence of trees. In a virtual simulator study, subjects ‘drove through’ four digitally created streets (one pair urban and one pair suburban). The two streets in each pair were identical except for the presence of relatively closely spaced mid-sized trees. The subjects were asked to rate their perception of roadway safety, and the sense of spatial definition.  The results indicated that suburban streets were perceived as the safest, followed by urban streets with trees. Urban streets without trees were perceived as the least safe. In both urban and suburban streets, the presence of trees was associated with a greater sense of ‘spatial edge’, which contributed to a greater perceived sense of safety. It was concluded that both city form (urban/suburban) and landscaping (the presence or absence of street trees) influenced the subjects’ perception of safety. However the presence of street trees had a greater impact on perceptions of safety than the surrounding land uses. Individual driving speeds were also significantly reduced when street trees were present in the suburban street, and both faster and slower drivers drove more slowly in the presence of trees. (The speed data for urban streets could not be successfully analysed due to the short block lengths). The authors concluded that drivers’ perception of safety had a significant relationship to their perception of the roadway edge, and the addition of kerbside trees significantly increased driver perception of that spatial edge.

The results of these studies indicate that there may in fact be a positive effect of having a well-defined roadside edge, which may lead to an overall decrease in run-off-the-roadway collisions with roadside objects. Street trees help to define the edge of the road space by providing a diverse visual edge that also is repetitively simple in colour, texture, and form (Naderi et al., 2008). (Lynch, 1960) had earlier theorized that a distinct roadway edge helps contribute to the ‘legibility’ of the city, fostering a feeling of familiarity and comfort. These findings suggest a different role for trees in urban and suburban streets, one that enhances traffic safety rather than threatening it.

4.8.4 Intersection sight lines

Memorable streets are often tree-lined, with trees at regular spacings, and planted all the way to the intersection corners (Jacobs, 1993). Howeversafety concernshave resulted in trees being set back long distances from intersections, to maintain sight lines fordrivers (Macdonald, 2008). Traffic engineering design manuals by authorities such as the American Association of State Highway and Transportation Officials (AASHTO) recommend designing intersections with clear sight triangles to improve a driver’s ability to see potential conflicts with other vehicles before entering the intersection.

In the United States, (MacDonald, 2008) has queried these restrictions for two reasons. Restricting trees may not solve visibility problems, as other objects can still block sight lines, including parked cars; and streets do not function as well as they should for pedestrians by providing comfort, legibility and shade. MacDonald points out that clear sight diagrams include unrealistic 2 dimensional representations of trees based on a tree canopy circle. This representation is unrealistic as street trees are usually pruned to create a raised canopy on a bare trunk. In reality, the part of the tree which will intrude on the driver’s cone of vision is the trunk rather than the canopy, a relatively thin vertical element. In addition, while eliminating trees at intersections, other urban elements such as street furniture and light poles may be allowed near intersections. In addition parked cars may block driver vision to a greater extent than tree trunks.MacDonald  has applied three dimensional modelling to simulate sight lines at typical urban intersections, varying the presence of street trees, parked cars and newspaper racks (Macdonald et al., 2006). The models were animated with moving cars and drive through simulations from the driver’s viewpoint. It was concluded that street trees, if properly selected, adequately spaced and pruned to branch high, do not create major visibility problems for drivers entering intersections. In fact parked cars, especially large 4WD ones, create substantially more visibility problems. The research suggests that street trees planted close to intersections, spaced as little as 8 metres apart, and pruned to 4.2 metres above the ground, do not constitute a visibility hazard on urban streets. MacDonald concludes that, although further research is required, the AASHTO guidelines need to be re-evaluated.

4.9 Air quality improvement

4.9.1 Introduction

Green infrastructure can play an important role in improving air quality in cities. Figure 24summarizes the main atmospheric benefits and air quality related ecosystem services provided by Green Infrastructure.

Figure 24: Summary of air quality benefits of Green Infrastructure. By author.

4.9.2 Air quality issues Air pollution

Poor air quality is an issue in many urban areas, due to a combination of population growth and urbanization, and increased pollutant emissions due to industrialization and the use of transport based fossil fuels (Treeconomics, 2011).The main pollutants of concern which are monitored by air quality authorities include (EPA, nd):

  • Nitrogen dioxide (NO2)
  • Fine particles-particulate matter (PM10)
  • Carbon monoxide (CO)
  • Ozone (O3)
  • Sulphur dioxide (SO2)
  • Lead (Pb) Air quality and urban heat island effects

Urban heat island effects increase overall electricity demand, as well as peak demand generally occurring on hot summer weekday afternoons. During extreme heat events (exacerbated by the urban heat island effect) the resulting demand for cooling can overload supply systems and lead to blackouts, or controlled blackouts to avoid power outages. Research shows that electricity demand for cooling increases by 1.5–2.0% for every 0.6°C increase in air temperatures suggesting that 5–10% of community demand for electricity is to compensate for the urban heat island effect (Akbari, 2005). Electricity supply typically relies on fossil fuel power plants, which in turn leads to an increase in air pollutant and greenhouse gas emissions. The primary pollutants from power plants include sulphur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), carbon monoxide(CO), and mercury (Hg). These are both harmful to human health, and contribute to complex air quality problems including the formation of ground-level ozone (smog), fine particulate matter, and acid rain. Increased use of fossil-fuel by power plants also increases emissions of greenhouse gases, such as carbon dioxide (CO2) which contribute to global climate change(EPA 2012). In addition to impacts on energy-related emissions, increased temperatures can also directly increase the rate of ground-level ozone formation (formed when NOx and volatile organic compounds (VOCs) react in the presence of sunlight and hot weather). In general more ground-level ozone will form as the environment becomes sunnier and hotter. Air quality impacts

The problems caused by poor air quality in urban areas are well known and include (i-Tree, 2010):

  • Impacts on human health
  • Damage to ecosystems and ecosystem processes
  • Damage to buildings
  • Reduced visibility

In recent years there has been considerable interest in the adverse health effects of exposure to air pollution. A number of epidemiological studies (EPA, nd) have shown links between;

  • Air pollution and asthma (Jalaludin et al., 2004),
  • Air pollution and attendances at emergency departments for cardiovascular disease in the elderly (Jalaludin et al., 2006).

Many of these epidemiological studies have been conducted in the US, and Europe. Researchers have observed that in general air quality in Australian cities is much better than that observed in many American and European cities (EPA, nd). However studies in Sydney (Morgan et al., 1998a; Morgan et al., 1998b) and Brisbane (Simpson et al., 1997) have indicated that levels of ambient air pollution in those cities have contributed to variations in daily mortality and hospital admissions for cardio-respiratory disease, and that the effects observed overseas do also occur here.The Victorian EPA conducted an epidemiological study to investigate the effects of air pollution on daily mortality in Melbourne from 1991 to 1996 (EPA, nd). The results showed that ambient air pollution in Melbourne was associated with increases in daily mortality. Although all of the air pollutants under consideration in this study (ozone, nitrogen dioxide, fine particles and carbon monoxide) were found to be associated with daily mortality, the strongest associations were for ozone and nitrogen dioxide. The main sources of these pollutants in Melbourne were motor vehicles and industrial activity. The report suggests that ‘strategies to reduce these pollutants are important to reduce the risk of adverse health effects arising from exposure’(EPA, nd) p.42. The results of the study were also found to be consistent with other studies conducted within Australia and overseas. Another Australian study (Peach ,1997) noted that overseas research has linked the levels of particulates, sulphur dioxide and ozone with increased morbidity and mortality.

4.9.3 Pollutant reduction by vegetation Role of vegetation

Studies have found links between urban tree cover and air quality (Escobedo et al., 2008).

One study indicated that a higher street tree density was associated with lower childhood asthma prevalence (Lovasi et al., 2008). A study in Santiago, Chile found that urban forestry may be effective in improving air quality, particularly in terms of removing atmospheric particulates (PM10) (Escobedo et al., 2008; Escobedo and Nowak, 2009). Because the filtering capacity of vegetation is closely linked to leaf area, trees with larger canopies can provide the most benefits (Treeconomics, 2011). One Australian study found that vegetation could limit CO2 fluxes to the atmosphere, caused by urban traffic (Coutts et al., 2007). Pollutant reduction mechanisms

A recognized ecosystem service provided by urban trees and vegetation is that of improving air quality in cities. The natural functions of urban trees are known to remove atmospheric pollutants,oxygenate the air, and absorb carbon dioxide through photosynthesis (Brack, 2002; Nowak et al., 2006).The natural functions of vegetation can directly and indirectly improve air quality in the following ways (Nowak, 1995):

  • Direct removal of pollutants through either:
    • Absorbing gaseous pollutants through the leaf surface (SO2, NO2).
    • Intercepting particulate matter on leaves (PM10).
  • Reducing air temperatures through shading and evapotranspiration, and thereby lowering ozone levels (O3).
  • Indirectly, by reducing air-conditioning use and related energy consumption in buildings (through shading of buildings, air temperature reduction and wind modification) leading to lower air pollutant emissions from power plants (known as ‘avoided emissions’).

Figure 25: Air quality improvement by trees. Source: (McPherson, 2010).

It must be recognised however that trees can also impact negatively on air quality through the emission of volatile organic compounds (VOC) and from emissions resulting from tree management activities.Trees emit volatile organic compounds that can contribute to ozone formation in the atmosphere (Chameides et al., 1988). However cumulative studies involving urban tree impacts on ozone have revealed that increased urban canopy cover, particularly with low VOC emitting species, leads to net reduced ozone concentrations in cities (Cardelino and Chameides, 1990; Taha, 1996; Nowak et al., 2000; Nowak and Dwyer, 2007).

Trees remove gaseous air pollution primarily by uptake via the leaf stomata, although some gases are removed by the plant surface. Once inside the leaf, gases diffuse into the intercellular spaces and may be absorbed by water films to form acids or react with the inner-leaf surfaces (Smith, 1990). Studies show that gaseous pollutants are absorbed by leaves and either metabolized or transferred to the soil by decay of leaf litter, which may be particularly important in streets with high traffic volumes(Nowak, 1994; Scott et al., 1998).

The leaves of trees also collect and trap airborne particles on their surfaces. Some particles can be absorbed into the tree, although most that are intercepted are retained on the plant surface. The intercepted particles however are often re-suspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fall, therefore vegetation may provide only a temporary retention site for many atmospheric particles (Nowak et al., 2006). Interestingly oxygenation by urban treesis only of limited value compared with the major global oxygen sources such as the oceans and forests (Nowak et al., 2007) and the most significant impacts on human health and environmental quality are through reductions in carbon dioxide and atmospheric pollutants (Nowak et al., 2002; Nowak et al., 2006; Nowak et al., 2007). Quantifying pollutant reduction benefits

The i-Tree tool has been used on a number of occasions to calculate pollutant removal by the urban forest in the US and elsewhere. For example pollution removal by trees in Washington was estimated using field data and recent pollution and weather data (i-Tree, 2010).  Pollution removal was greatest for O3, and it is estimated that trees studied removed 492 tons of air pollution (CO, NO2, O3, PM10, SO2) per year with an associated value of $2.30 million, based on estimated national median externality costs associated with pollutants (Murray et al., 1994).

Figure 26: Pollution removal and associated value for trees in Washington (line graph is value). Source: (i-Tree, 2010)p.7.

4.9.4 Traffic generated pollutants Health impacts

In recent years increasing numbers of studies have linked proximity to roads with adverse health effects from exposure to traffic-generated pollutants. There is a general consensus that people spending significant amounts of time near major roads face increased risks for several health effects (Health Effects Institute, 2009). These effects can be attributed to particulate matter, gaseous pollutants, and air toxins. For particulate matter the main constituents of concern include ultrafine particles, coarse particles, metal constituents, and organic compounds (Baldauf et al., 2011).

A recent article in the US described how roadway design, including the presence of roadside vegetation, may provide a means of mitigating air pollutant concentrations near roads (Baldauf et al., 2009). In 2010, a multidisciplinary group of researchers and policy-makers in the US met to discuss the state-of-the-science regarding the potential role of vegetation in mitigating air quality impacts from traffic emissions (Baldauf et al., 2011). It was found that vegetation near roads can play a number of pollutant mitigating roles outlined in the following sections. Dispersion-only-based impacts

Field and wind tunnel studies have shown that dense vegetation barriers can play a similar role to solid barriers in dispersing near-road pollutant concentrations of inert gases such as carbon monoxide. Wind tunnel and field tracer study have revealed consistent reductions in ground-level concentrations behind barriers relative to situations with no barriers (Heist et al., 2009; Finn et al., 2010). The presence of a barrier was found to lead to an increase in vertical mixing, resulting in lower behind-barrier concentrations at ground level. Enhanced capture of particulate matter

Field and wind tunnel studies have investigated the potential for enhanced capture of particulate matter by vegetation. Generally, these studies have shown decreases in concentrations of ultrafineand coarse particles, with limited reductions measured for PM 2.5 (Baldauf et al., 2008; Fujii et al., 2008). It has been noted, however that under certain climatic conditions PM concentrations could be higher behind a vegetative barrier than on the roadside (Baldauf et al., 2008). Complex flow patterns

Modelling studies indicate that roadside barriers (including vegetated barriers) are associated with complex flow patterns. Two recent modelling studies have simulated the impact of roadside vegetation on near-road air quality (Zhu et al., 2002; Heist et al., 2009). A 3D model revealed significant reductions in down-wind inert pollutant concentrations in the presence of the barrier due to enhanced turbulence and mixing (Heist et al., 2009). A 2D model suggested that, under low wind speed conditions, concentrations can be higher on the downwind side of a vegetative or solid barrier, at further distances from the road, than would have occurred without a barrier (Zhu et al., 2002).In this situation the traffic emissions were forced up and over the barrier, with the plume remaining mainly intact until returning to ground-level, leading to higher concentrations further downwind. Air quality in dense canyons

Modelling vegetation in extremely dense development, such as street canyons, has shown generally lower concentrations on the windward side, but higher concentrations on the leeward side of the street canyon (Gromke and Ruck, 2007; Buccolieri et al., 2009).Researchers generally agreethat local meteorology and site design are critical factors affecting air quality impacts around roadside vegetative barriers. (Pugh et al., 2012)observed that street-level concentrations of nitrogen dioxide (NO2) and particulate matter (PM) exceed public health standards in many cities, causing increased mortality and morbidity. These can be reduced in three ways:

  • Controlling emissions.
  • Increasing dispersion of pollutants.
  • Increasing deposition rates of pollutants.

According to Pugh, in the US little attention has been paid to increasing deposition rates as a pollution control method (Pugh et al., 2012). Both NO2 and PM are deposited onto surfaces at rates which vary according to the nature of the surface, and deposition rates to vegetation have been found to be much higher than those to hard surfaces. A recent study by (Pugh et al., 2012) at Lancaster University has found that adding trees, bushes, green walls, or even ivy or other creeping vines, can reduce street-level nitrogen dioxide (NO2) and particulate matter (PM) by eight times more than previously thought. Previous city-scale studies have suggested that deposition to vegetation results in only a very minor air quality improvements of around 5%. However few studies have taken full account of the interplay between urban form and vegetation, specifically the enhanced ‘residence time’ of air in street canyons.The study showed that increasing deposition through the planting of vegetation in street canyons (such as grass, climbing ivy and other plants) can reduce street-level concentrations by as much as 40% for NO2 and 60% for PM, much more than previously believed (Pugh et al., 2012). The authors even suggest constructing plant-covered ‘green billboards’ in such urban canyons to increase the amount of foliage. Trees were also shown to be effective, but only if care is taken to avoid trapping pollutants beneath their crowns.The authors concluded that judicious use of vegetation can create an efficient urban pollutant filter, yielding rapid and sustained improvements in street-level air quality in dense urban areas.

Figure 27:  Vegetation cover and pollutant reduction in a stret canyon. Source:(Pugh et al., 2012).


4.9.5 Guidelines for Air Quality Improvement

Table 4 presents a number of urban forest management strategies to help improve air quality  as recommended by (Nowak, 2000).

Table 4: Urban forest management strategies to help improve air quality. Source:(i-Tree, 2010).



Increase the number of healthy trees

Increase pollution removal

Sustain existing tree cover

Maintain pollution removal levels

Maximize use of low VOC-emitting trees

Reduces ozone and carbon monoxide formation

Sustain large, healthy trees

Large trees have greatest per-tree effects

Use long-lived trees

Reduce long-term pollutant emissions from planting and removal

Use low maintenance trees

Reduce pollutants emissions from maintenance activities

Reduce fossil fuel use in maintaining vegetation

Reduce pollutant emissions

Plant trees in energy conserving locations

Reduce pollutant emissions from power plants

Plant trees to shade parked cars

Reduce vehicular VOC emissions

Supply ample water to vegetation

Enhance pollution removal and temperature reduction

Plant trees in polluted or heavily populated areas

Maximizes tree air quality benefits

Avoid pollutant-sensitive species

Improve tree health

Utilize evergreen trees for particulate matter

Year-round removal of particles


4.10 Noise abatement

4.10.1 Overview

It is known that noise from traffic and other sources can decrease quality of life in cities. Ambient sound has also been shown to affect pedestrian behaviour, and as traffic noise increases pedestrians remember fewer details in the physical environment, walk faster and look around less often (Zacharias, 2001). Sound scattering by vegetation is complex and depends on a number of factors (Martens, 1981). Recent research has investigated two aspects of noise reduction: the study of vegetative barriers as measurable traffic noise attenuators; and the psychological or perceptual role of vegetation.

4.10.2 Vegetation buffers

In highway situations properly designed plantings of belts of trees and shrubs can reduce traffic noise to adjacent residential areas although not as effectively as solid barriers such as noise walls. However, to be an effective noise attenuator or buffer, plantings must be of sufficient width (generally over 10 metres) to achieve reductions in the order of 3-8 decibels (Fang and Ling, 2003). Such measures however, are not readily applicable in narrower urban streets. The level of noise reduction will also depend on the type and arrangements of planting, as well as the contribution of soil type to sound absorption (Mulligan et al., 1982; Anderson et al., 1984; Fang and Ling, 2003). One study quantified relationships between noise attenuation and the type and height of vegetation. Density, height, length and width of tree belts were found to be the most effective factors, rather than leaf size or branching characteristics (Hayaashi et al., 1980). Wide plantings of tall dense trees combined with softground surfaces have been found to reduce apparent loudness by 50 percent or more (6-10 decibels) (Cook, 1978). In many cases the high cost of conventional highway noise abatement measures (such as free-standing walls) has made mitigation of impacted sites economically not feasible, and a solution that may prove more economically acceptable for such sites is the use of strategically planted evergreen vegetation (Harris and Cohn, 1985).

Recent research by (Van Renterghema et al, 2013) entitled ‘The Potential of Building Envelope Greening to Achieve Quietness’ shows that urban greening can play a role in quieting urban areas. The study focussed on road traffic noise propagation towards the traffic-free sides of inner-city buildings (i.e. urban courtyards). Preserving quietness at such locations has been shown to be beneficial for the health and well-being of users. The results in the study show that green roofs have the highest potential to enhance quietness in courtyards. Favourable combinations of roof shape and green roofs were also identified. Vegetated façades were most efficient when applied to narrow city canyons with otherwise acoustically hard façade materials. Greening of the upper storeys in the street and (full) façades in the courtyard itself was most efficient to achieve noise reduction. Low-height roof screens were shown to be effective when multiple screens are placed, but only on condition that their faces are absorbing. The combination of different greening measures resulted in a lower combined effect than when the separate effects would have been linearly added. The researchers considered that the combination of green roofs or wall vegetation with roof screens appeared the most useful strategy. According to the researchers, greening could be used to limit noise from other sources, such as air conditioning units, although the study focused solely on traffic noise.

4.10.3 Psychological roles

It has been shown that trees in urban streets provide only minor noise reduction benefits, unless associated with earth mounding or noise walls. Street trees however can also play an important psychological role by ‘masking’ traffic noise with their rustling leaves (O'Brien, 1993). Noise-masking is a useful technique for treating the problem of noise that is simply annoying rather than excessively loud (Heisler, 1974). Research has also shown that the visual and acoustic attributes of urban vegetation may interact to alter the perception and evaluation of sound (Mulligan et al., 1982). For example, subjects have sometimes reported a noise reduction as a result of thin planting strips, and even hedges that were too sparse to have any physical impact on sound transmission. Other field tests have indicated that the environment in which the sounds are perceived may have an impact on the perception of noise levels. People perceived the same level of noise as higher in green settings than in more typical city settings, which indicates an adaption to certain sounds in different settings (Anderson et al., 1984). In one study researchers found that vegetative barriers created a perceived attenuation of road noise of 3-5 decibels, and concluded that visualization of the noise source directly affected perceived sound levels (Ishii, 1994).

4.11 Summary

  • A diverse body of research shows that Green Infrastructure can enhance the liveability of cities and the ‘quality of life’ in urban areas.
  • This is a broad topic and includes a range of the less easily measured aspects of liveability including cultural values, aesthetics and other factors that contribute to urban amenity including the ‘walkability’ of streets and air quality.
  • Green Infrastructure including urban greening in various forms can enhance liveability, and urban trees have been identified as making a significant contribution to urban amenity.
  • Green infrastructure has been identified as one of the key principles for enhancing liveability in the modern densified city.

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