When one thinks of sustainable infrastructure, roads are one of the last things that generally come to mind. These lifeless, impermeable surfaces are used by greenhouse gas belching vehicles and contribute to heating up our urban environments, making them the enemy of many a dedicated “greenie”. However roads could be made a lot more sustainable than they are, and given that they cover around one third of the land area in the City of Sydney Local Government Area there is a lot to be gained by making improvements in this field.
After extensive consultation with urban sustainability expert Michael Mobbs I’ve come up with three key actions to make our roads more sustainable. I’ll be trying over the coming weeks and months to persuade council to set up a demonstration sustainable road where these schemes are implemented and that, if successful, will serve as a model for the more widespread greening of roads across Sydney and elsewhere.
The image gives an indication of a range of things that can be done to make our streets more sustainable while details of three key actions in this process are outlined below. (If you have trouble viewing the image, click here sustainable-road-design.pdf)
The big problem with roads, aside from the cars that use them, is how unnatural their surfaces are. The asphalt that roads are made from doesn’t absorb water like soil in bushland does; rather it lets it all run off. While roads don’t absorb water they do absorb far more heat than most natural surfaces and as a result of these two factors they make a significant contribution to the ‘urban heat island effect’.
The urban heat island effect is the phenomenon whereby cities are up to 7oC warmer than corresponding undeveloped areas. 7 oC can be the difference between a pleasant 25 oC and a scorching 32 oC. As well as making our cities less pleasant the urban heat island effect results in big increases in power bills and greenhouse gas emissions as a result of greater air-conditioner use and makes a significant contribution to the number of heat related illnesses and deaths suffered every year.
One obvious way to help counter this effect is to make our urban environments more closely resemble their cooler natural counterparts and an easy way to do this is through the use of roadside vegetation.
Vegetation can help to cool an urban environment in many ways. Firstly, trees provide shade that help people, and highly heat absorbent roads, stay out of direct sunlight. Secondly, trees contribute to evaporative cooling as a result of water vapour being emitted from leaves.
Finally, vegetation has a much higher level of a property called albedo than roads do. Albedo is a measure of how much light a surface reflects – the more reflective a surface is the less it warms up in the sun and the more sunlight gets reflected back out into space.
As a result of all these cooling factors vegetation can have a significant mitigating effect on the urban heat island. Studies have shown that an urban landscape with 30% tree cover can be up to 6 oC cooler than the same area would be without any trees at all .
It addition to the various ways in which trees cool our urban environments they also play an important role in absorbing carbon dioxide emissions, providing wildlife habitat and just looking beautiful. Planting of fruit trees or other plants which produce edible crops can also contribute to the overall health of urban populations by providing a source of fresh food and have a positive environmental impact by reducing the distance that food must travel to reach consumers and the amount of land that must be cleared for agriculture.
The City of Sydney is already well ahead of many world cities in terms of level of tree cover but I will be asking Council to investigate the possibility of planting more trees on median strips on wide streets and other underutilised areas in order to make a contribution to improving the sustainability of our roads.
Lighter coloured road surfaces
Another way to improve the sustainability of roads and help counter the urban heat island effect through the use of higher albedo surfaces is to change the way roads themselves are constructed. Instead of using black asphalt that makes up most roads (low albedo) the City could use lighter coloured concrete (higher albedo).
As roads cover about one third of the land in the City of Sydney local government area replacing their black asphalt surfaces with lighter coloured concrete (which reflects 30% more heat) could effectively increase the amount of heat reflected by the City by around 10%.
Modelling carried out on Los Angeles, which has a similar climate to Sydney, showed that such an increase in reflected heat could reduce urban temperatures by up to 3 oC . This would make our City a much more pleasant place to be in summer, reduce the incidence of heat related illness and, if used in conjunction with increased vegetation, potentially contribute to a reduction in energy used for air-conditioning of up to 61% .
More reflective road surfaces can be beneficial not just in terms of heat but also light. While the City is currently undergoing a range of trials into saving energy through more efficient street lighting these trials are focusing entirely on the structures that emit the light, the lamps themselves, and not on the areas that receive it.
It is obvious that a lamp illuminating an area with light coloured surfaces will make the area brighter than the same lamp illuminating an area with dark coloured surfaces. This means that if the dark asphalt roads and footpaths of the City were replaced with lighter coloured concrete surfaces the same illumination we have now could be achieved with less energy use.
Studies from the USA have found that this could effectively reduce lighting costs by almost one third as a result of decreases in energy use and maintenance costs . Given that the city currently spends around $5.5 million per year to power and maintain public lighting it is obvious that there are significant economic, as well as environmental, benefits in switching to lighter coloured paving surfaces.
As touched on earlier, part of the reason urban environments are generally warmer than natural ones is that the former don’t retain water. While in a bushland environment most rain is absorbed by soil and taken up by plants in cities the bulk of it just runs off. In order to combat the urban heat island effect we need to keep as much water as possible where it falls so that it can cool the environment through evaporation and so that vegetation can be grown without using up our valuable drinking water supply.
Preventing runoff also has the secondary benefit of reducing the amount of urban rubbish and pollutants washed into our waterways.
One means of keeping rainwater in the urban environment is through the use of rain gardens. Rain gardens are like ordinary gardens except that they are planted in gutters to absorb the rainwater that would otherwise run off. The City has set up a number of these but I feel they are working towards a sub-optimal goal.
One of the stated aims of the City’s rain gardens is to clean urban stormwater run-off. However I believe a more practical and achievable goal for the rain gardens should be to stop urban stormwater run-off entirely in all but the biggest storm events. The rain gardens need to be built in such a way that stormwater is slowed down sufficiently to be entirely absorbed by the soil of the garden. This doesn’t happen with the current rain gardens and water flowing at high speed washes soil, mulch and gravel into the drain, destroying the garden’s viability and adding potentially more contamination to the harbour than it removes.
If the demonstration sustainable street goes ahead I’ll be seeking to have the design of the rain gardens reviewed so that they can more effectively retain water, helping to cool our streets as well as keep urban contaminants out of our waterways.
Post script: At the meeting of Council on 6th of December, following the publication of this article, Council CEO Monica Barone presented this memo to Council, calling for a sustainable demonstration street to be set up in Myrtle St, Chippendale. This motion was passed by Council and I will now very much look forward to seeing how this project pans out over the coming months and years.
 Haida Taha (1997)-Urban climates and heat islands: albedo, evapotranspiration and anthropogenic heat. Energy and Buildings, volume 25, page 99-103.
 Arthur Rosenfeld et al (1998)- Cool communities: strategies for heat island mitigation and smog reduction. Energy and Buildings, volume 28, page 51-62.
 John Gadja and Martha VanGeem (2001) - A Comparison of Six Environmental Impacts of Portland Cement Concrete and Asphalt Cement Concrete Pavements.