Street trees for complete streets: getting people to walk more

The Heart Foundation supports the creation of healthy built environments that encourage more people to walk more often, as a part of a multilevel approach to reducing the risk of heart disease.

Population growth concentrated in cities and urban areas is inevitable, and increasing residential density should be part of the overall plan for South Australian population growth. As we move towards more medium density housing, the success lies in convincing the community of the necessity of smaller private spaces and the challenge is to expand and improve public open spaces nearer to where we live.[i]

The intensification of development in Greater Adelaide’s urban areas requires that built environment professionals and developers consider the role of streetscapes as not solely the domain of motor vehicles but also for pedestrians and cyclists. Conventional streets favour and prioritise the movement of vehicles, with the quantity and quality of space for people on foot often only considered as an afterthought.[ii]

Instead, to support walking, the role of the street must be re-considered as a place to be somewhere, not just get somewhere. Our streets are becoming increasingly important as public spaces for social and commercial interaction.

Streetscape amenity and the perception of the green-ness of a neighbourhood affect the rates of walking, particularly recreational walking. Street trees are a necessary part of this streetscape and contribute value at the social, environmental and economic levels.

It seems obvious, but street trees are the underappreciated asset that can have a significant impact on our street experience. Wouldn’t we all like to walk or ride down a street that has a significant tree canopy and landscaping, rather than bare slabs, exposed to the elements?

But concerns about root systems, drought tolerance, maintenance or dropping seeds/fruits have seen reluctance to invest and removal rather than integration into development. For councils deciding on where to direct expenditure in order to improve a street, planting trees will give the best return as trees can transform a street more easily than any other physical improvement.

Streets trees can encourage walking and promote wellbeing in several ways: [iii]

  • By providing facilitative settings encouraging people to walk for both exercise and transport; -
  • By facilitating social interaction and a sense of community; -
  • Trees can aid the healing process for those recovering from stress related illnesses; -
  • 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. The perception of safety is an important component of walkability, and trees create a protective barrier which reduces the risk of being hit by a ‘run-off-the-road’ vehicle.[iv]–[v]
  • By mitigating the adverse effects created by urban heat islands;
  • Trees (low-allergen) absorb considerable quantities of airborne pollutants and the resulting cleaner air cuts childhood asthma levels.[vi] 

Trees contribute to the textural detail missing from architecture. Light filtered through trees gives life to space. Manipulation of light and shade transforms stone, asphalt and concrete into tapestries of sunlight and shadow.[vii]

The Heart Foundation’s Healthy By Design SA guidelines aim to assist planners, urban designers and related professionals to design healthy urban environments that enable people to make healthy lifestyle choices and in particular incorporate walking and cycling into their daily routine. The guide suggest that planners use tree planting and landscaping to contribute to the functionality of streets and open spaces, improve the microclimate and create attractive and legible routes and spaces that encourage active use.[viii]

Trees deliver ‘cool’ streets

Trees and shade in the street are critical in creating an environment that people want to walk in, especially for recreation. The benefits for walkers are both aesthetic and practical, with street trees providing shelter from the sun and, to some extent, rain.[ix] An Australian study found the presence of trees providing shade in open spaces was positively associated with an increased likelihood of being active.[x]

A study looking at the effect of urban green on walking[xi] found that the density of single street trees, within a one kilometre network buffer, local street level connectivity, and proximity to service destinations are all independently positively associated with the study participants desire to walk. The study confirmed the general hypothesis that streets lined with trees are associated with positive perceptions of greenness and street environment quality, and therefore provide an incentive to walk. 

Lee and Moulden[xii] 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 variable 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 big tree argument

Geiger advocates the case for growing large trees in cities.[xiii] Large, mature trees are considered to deliver more significant benefits than smaller stature trees. Therefore, large tree species should be planted, and trees should be allowed to grow to maturity to maximise their benefits. For example, large trees provide greater benefits of improved shade, water quality and air quality than smaller trees.[xiv] Large trees out-perform small trees in moderating air temperatures, blocking UV radiation, conserving energy, sequestering carbon and reducing air pollution, in a manner directly related to the size of the tree canopy.[xv] A study by McPherson[xvi] estimated that a large tree with a height of 14 metres provides three times the annual environmental benefits of a similarly aged 7 metre high tree, and that the value of benefits increases faster than the costs of managing a larger tree.

Larger trees also have greater visual presence, and are often more highly valued by residents, especially where ‘canopy closure’ over the street is achieved.[xvii][xviii][xix] In one study the single largest factor in determining the attractiveness of a street scene was the size of the trees and their canopies. This was supported by a study in which there was a preference for large canopied trees in a tree replacement program.[xx] 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 our fast changing society.[xxi]

Accommodating trees in streets

Street trees have traditionally been planted in the road verge (the zone between the footpath and roadway) however they can also be planted in the adjacent parking lane or street median. To be most effective, street trees and associated landscaping must be carefully integrated with other street functions. The US Institute of Transportation Engineers has published guidelines for Designing Walkable Urban Thoroughfares: A Context Sensitive Approach.[xxii]

The guidelines recommend designing urban streets based on the delineation of four distinct zones:

  1. Edge zone-between the kerb and furnishing zone, providing separation between objects and activities in the street side and vehicles in the travel way.

  2. Furnishing zone-the area which provides a buffer between pedestrians and vehicles, containing landscaping, street furniture and infrastructure.

  3. Throughway zone-the walking zone providing for the clear movement of pedestrians, including minimum disability access requirements.

  4. Frontage zone-the space between the throughway zone and building frontage/property line, containing street furniture, outdoor eating etc.

Landscaping is typically located in the furnishing zone. The width of the street side landscaped strip is recommended to be a minimum of 1.5 metres, with 2.4 metres preferred, to support healthy tree growth. Trees can also be planted in ‘protuberances’ in the parking zone, which helps reduce the perceived width of the street.

The Bowden Urban Village in Adelaide, South Australia, used innovative street planting for creating cooler walkable environments as well as achieving lower vehicle speeds.[xxiii] To achieve this, trees were planted along the edges of pedestrian environments and in the middle of the street, and landscaping areas protruded unevenly into the vehicle space. .

Key actions

Improvements to the micro-level environment have the potential to promote walking. Public realm and streetscape features that can improve the amenity of a neighbourhood include street lighting, shade trees, and the installation and maintenance of footpaths and street-crossing aids. The introduction of walking and cycling infrastructure as well as traffic calming and other traffic diversions also have the potential to help to encourage local walking as well as cycling.[xxiv]

Healthy By Design SA suggests:

  • Consider the use of landscaping and trees to perform functions that may normally be performed by engineered grey infrastructure.[xxv]
  • Use clear-stemmed trees with high canopies, or prune trees to create a raised canopy on a bare trunk to maintain clear sightlines and surveillance.
  • Plant large trees and allow trees grow to maturity to maximise their benefits and improve the microclimate. Large trees outperform small trees in moderating air temperatures, blocking UV radiation, conserving energy, sequestering carbon and reducing air pollution. [xxvi]
  • Use tree planting to reduce the ‘urban heat island effect’. Trees and other vegetation can modify urban microclimates and help reduce the urban heat island effect by shading urban surfaces from solar radiation and through evapotranspiration, which has a cooling and humidifying effect on the air.[xxvii][xxviii][xxix]
  • Select sustainable trees and plants that are non-invasive and suited to the local conditions, require reduced resources and inputs of energy and materials, enhance biodiversity and create habitats, and avoid the use of harmful chemicals. Consider deciduous.
  • Where appropriate, label trees and vegetation to add interest. Providing positive aesthetics, points of interest and natural features increases the walkability of routes.[xxx]

Acknowledgements

The Heart Foundation would like to acknowledge that the source of the sections “the big tree argument” and “accommodating trees in streets” is from an unpublished report commissioned by the Heart Foundation SA in 2014, and written by Dr Martin Ely, entitled: Building the case for the role of landscaping in urban street design.

Related links

Quality green space, literature review

Making the case for investment in street trees

References

[i] Udell T, et al. Does density matter? The role of density in creating walkable neighbourhoods. Melbourne: National Heart Foundation of Australia. 2014.

[ii] Government of South Australia, National Heart Foundation of Australia. Streets for People. A Compendium for South Australian Practice. Adelaide 2012.

[iii] Heart Foundation SA: Position snapshot: Making the case for investment in street trees and landscaping in urban environments. 2012.

[iv] CTRE. Clear Zone: A synthesis of practice and an evaluation of the benefits of meeting the 10 foot Clear Zone goal on urban streets. Ames, IA: Cetre For Transport Education and Research, Iowa State University, 2008

[v] Jacobs AB. Great Streets: Massachusetts Institute of Technology; 1993.

[vi] Lovasi G, et al. Children living in areas with more street trees have lower prevelance of asthma. J epidemiol Community Health. 2008;62:647-9.

[vii] Ewing R, Handy S. Measuring the unmeasurable: urban design qualities related to walkabilty. J of Urban Design. 2009;14(1):65-84.

[viii] National Heart Foundation of Australia. Healthy By Design SA: A guide to planning, designing and developing healthy urban environments in South Australia. 2012

[ix] Vic Walks http://www.victoriawalks.org.au/trees/

[x] Timperio, A., et al. Features of public open spaces and physical activity among children: Findings from the CLAN study. Preventive Medicine, 2008; 47(5), 514–518.

[xi] Sarkar, C et al. Exploring associations between urban green, street design and walking: Results from the Greater London boroughs. Landscape and Urban Planning 2015; 143.

[xii] Lee, C and A Moulden. Correlates of walking for transportation or recreation purposes. Journal of Physical Activity 2006; 3(1): 77-98

[xiii] Geiger, J. R., et al. (2004). The Large Tree Argument – The case for large-stature trees vs. small-stature trees. Research summary. Davis, CA, Center for Urban Forest Research, Pacific Southwest Research Station, USDA Forest Service.: 8.

[xiv] McPherson, J. R., et al. (2005). Municipal Forest Benefits and Costs in Five US Cities. Journal of Forestry(December).

[xv] Nowak, D. J. (2004). Assessing environmental functions and values of veteran trees. Proceedings of the International Conference on the Protection and Exploitation of Veteran Trees, Torino, Italy.

[xvi] McPherson, E. G. (2005). “Trees with benefits.” American Nurseryman 201(7). [xvii] Kalmbach, K. L. and J. J. Kielbaso. Resident attitudes toward selected characteristics of street tree planting. Journal of Arboriculture. 1979; 5(6): 124-129.

[xviii] Schroeder, H. W. and W. N. Cannon. The aesthetic contribution of trees to residential streets in Ohio towns. Journal of Arboriculture. 1983; 9: 237-243.

[xix] Sommer, R., et al. Household evaluation of two street tree species. Journal of Arboriculture. 1989; 15: 99-103.

[xx] Heimlich, J., et al. Attitudes of residents toward street trees on four streets in Toledo, Ohio, U.S. before removal of Ash trees from Emerald Ash borer.” Journal of Arboriculture and Urban Forestry 34(1): 47-53.

[xxi] Dwyer, J. F., et al. (2003). “Sustaining urban forests. Journal of Arboriculture. 2008; 29(1): 49-55.

[xxii] Institute of Transportation Engineers. Designing Walkable Urban Thoroughfares: A Context Sensitive Approach. Publication No. RP-036A. Washington D.C. 2010.

[xxiii] Land Management Corporation; all street sections and photomontages by TCL and Jensen Planning. Bowden Urban Village, Adelaide, South Australia.

[xxiv] National Heart Foundation of Australia. Position Statement: The built environment and walking. 2009.

[xxv] National Heart Foundation, Ely M. Building the case for the role of landscaping in urban street design. Unpublished report for the National Heart Foundation, 2012.

[xxvi] Nowak DJ. Assessing environmental functions and values of veteran trees. In: Nicolotti G, Gonthier P (eds). Proceedings of the International Conference on the Protection and Exploitation of Veteran Trees. Torino, Italy: Regione Piemonte and Universita di Torino, 2004

[xxvii] Akbari H, Pomerantz M, Taha H. Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy 2001; 70(3):295–310.

[xxviii] McPherson EG, Herrington LP, Heisler GM. Impacts of vegetation on residential heating and cooling. Energy and Buildings 1988; 12:41–51

[xxix] Georgi NJ, Zafiriadis K. The impact of park trees on microclimate in urban areas. Urban Ecosystems 2006; 9(3):195.

[xxx] Heart Foundation WA. Healthy by Design Scoping Report, 2011.