Rethinking roads as green corridors

By Paul Geehan, Technical Director, Design + Planning

A city’s roads are essentially public space dedicated to a singular use – vehicular movement. And the primary objective of road design guidelines is the safe and efficient use of road corridors by vehicles. When we talk about corridor landscapes, it tends to be in terms of their impact upon road user efficiency and safety, not their amenity value.

Even before the current health pandemic, the number of people who regularly cycled or walked to work in Australia has been steadily increasing. The densification of our cities, especially on the east coast has made active transport a much more attractive option for many.

Current health concerns around crowded public transit have accelerated the cycling trend for those who are fortunate enough to live close enough to their workplace. The pop-up cycleways in Sydney have created a safer corridor and we have seen more families cycling together, not just Lycra-clad commuters.

Unfortunately, this period has highlighted disadvantages for those who live much further out and who don’t have alternatives other than public transport or jumping back in cars. For better or for worse, commuting may never be the same again.

We will eventually overcome coronavirus. However, the impact of global warming on our cities will remain.

As Australia emerges from winter and the heat builds towards the crescendo of the hottest months, the urban forest is one of the most effective responses to reducing city heat and encouraging active transport. Tree-lined streets that reduce heat stress and provide safe, shaded pedestrian and cycle routes are no longer a ‘nice to have’ option, they are a necessity.

Many government jurisdictions now have urban forest strategies in place, which outline how to achieve canopy cover through development controls and incentives. But these strategies wrongly assume that successful urban forest strategies can be achieved within the highly contested public realm, much of which has been dedicated to the provision of utilities such as telecoms, water, and power. All jurisdictions are severely limited by the competition for space within the urban street corridor, regardless of technical merit and a genuine will to succeed.

One study into the health impacts of urban vegetation found that by doubling the leaf canopy there would be up to 28 per cent fewer heat-related deaths. Apart from the human health benefits, tree canopy also extends the life of the road it shades.

AECOM’s Green Infrastructure report in 2017 found that in a Sydney street, the surface temperature of concrete and asphalt was at least 14 degrees cooler in the shade. The higher surface temperature on unshaded roads makes walking and cycling unpleasant, but also degrades the asphalt quicker.

If we genuinely want to create an urban forest to cool our cities, we must value the contribution of green infrastructure and address the competition for space between utilities (above and below ground), and road design requirements such as ‘clear zones’.

Green corridors

When it comes to road design and layout, trees are considered ‘non-frangible’ or unbreakable objects, excluded from ‘clear zones’ for safety reasons. Current road design guidelines limit or prevent the installation of street trees hampering any urban forest strategy, as citizen safety always trumps amenity.

Beautiful tree-lined boulevards that manage to balance urban canopy for pedestrians and cyclists with vehicular movement can be achieved. The evidence can be found in historic cities with grand boulevards like London and Paris, that have benefitted from a legacy of ambitious tree planting that pre-dates the prevalence of buried services.

Where street trees are promoted, they must compete for space with utilities. For example, the New South Wales Street Openings Conference of 1969 was a worthy attempt to coordinate the arrangement of utilities within the street corridor and successfully ensured a fair allocation of space within the street cross-section, and a code of conduct for trenching and reinstatement. The latest 2018 iteration of the Street Openings Conference acknowledges trees have a place within the street however, the typical cross-sections allocate no space for trees. Infrastructure providers have a separate remit that arguably, despite our best efforts, limits our ability to provide street trees.

We can’t consign this impasse to the too hard basket – climate change and impacts of coronavirus demand that we don’t. All players in the public realm must coordinate efforts to find a place for full canopy trees within our road corridors. It is time for a serious and coordinated green infrastructure strategy that deals with the real issues on the ground rather than concepts.

We need a multidisciplinary approach to street and precinct design, bringing road engineers and urban designers together at concept stage to reserve space for trees as part of a clear vision. This is true for developments within the existing urban fabric, but equally for greenfield developments where establishing the street tree master plan, in coordination with the early street layouts, should be key in subdivisions and then the basis for coordinating utility reticulation.

As we look forward to post-pandemic life, now is the time for a coordinated, collaborative and national approach to street trees and urban forest planning, based upon an appreciation of the critical value of green infrastructure for the future liveability of our cities and suburbs.

Paul Geehan is a Technical Director in the Design and Planning team at AECOM and is based in Sydney.

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