From little things: More than a third of public green space is road verge

Abstract

Road easements are ubiquitous in urban landscapes but the green space they provide has been largely ignored, even though it represents a significant proportion of total urban green space. In this study, the extent and spatial distribution of green space within road easements are compared with private residential gardens and other public green space in Melbourne, Australia. Ground-surface permeability to water data and aerial photography were used to identify and quantify the extent of green space within 47 randomly selected neighbourhoods. Road easement green space constituted 7.0% of land cover and a surprisingly high 36.7% of public green space. The percentage of the road easement that was green space was positively correlated with date of neighbourhood development, residential parcel size, footpath absence and social disadvantage. The percentage of residential parcels that was private garden was positively correlated with residential parcel size, but negatively correlated with road density, and positively correlated with date of neighbourhood development as date increased to the 1950s and then negatively correlated for the following decades. Streets with a greater percentage of road easement green space were associated with residential parcels that had a greater percentage of garden. The potential for road easement green space to provide for the biodiversity, ecosystem function and human amenity now being demanded from urban green spaces is much greater than previously thought. Planners, urban foresters, landscape architects, engineers and ecologists need to work together to reconsider the ecological and greening roles of existing road easements and consider this in future road designs. Our data suggests that the roles that residents, municipal governments and road authorities can play will vary across developments, with each presenting specific opportunities to improve the benefits flowing from road easement green space.

Introduction

Road easements are a ubiquitous component of urban form that fulfil many functions, including the transport of goods, pedestrian, cycle and vehicle movement, the collection of storm water and carrying overhead and underground utilities. They also contribute to the urban green space network because vegetation is common within the road easement. Road verges (also called the nature strip in Australia and, variously, tree lawn, parking strip, boulevard or sidewalk buffer in North America), along with roundabouts, outstands, median strips and traffic islands, are often-vegetated elements of the road easement. We use the term ‘road easement green space’ to describe the road verge together with these additional elements. Road easement green space is usually mown grass, perhaps with understorey, and often includes street trees.

Road easement green space provides many benefits, well summarised by Säumel et al. (2015), including regulating temperature (Gillner et al., 2015, Sanusi et al., 2015); mitigating stormwater runoff (Breen et al., 2004, Armson et al., 2013); and mitigating air pollution (Vailshery et al., 2013, Leonard et al., 2016). It can include remnant vegetation, including endangered species and vegetation communities (McDougall, 1987, Lorimer et al., 1997, Lorimer, 2006), and can include numerous distinct vegetation communities (Cilliers and Bredenkamp, 2000). The road easement network can be important in providing connectivity between patches of larger habitat (e.g. parks) for a range of fauna (Fernández-Juricic and Jokimäki, 2001, Oprea et al., 2009, Munshi-South, 2012) and habitat for many taxa including arthropods (Schaffers et al., 2012) and moths and butterflies (Saarinen et al., 2005). The human health benefits of road easement green space include promoting mental health (Bratman et al., 2012), reducing depression (Taylor et al., 2015) and reducing early childhood asthma (Lovasi et al., 2008). Tree and shrub plantings improve walkability (Bosselmann et al., 1984, Vich et al., 2019), which has numerous health benefits, such as reducing obesity (Lachowycz and Jones, 2011), all well summarised in Frumkin et al. (2017). Street greenery also improves social cohesion (Jacobs, 1961, De Vries et al., 2013) and is positively associated with safety, comfort, sensory pleasure and sense of belonging (Alfonzo, 2005, Southworth, 2005, Mehta, 2008).

Despite its narrow and fragmented nature, road easement green space's contribution to urban green space is likely to be substantial. We could find only one study that quantified its extent and one other study from which extent could be estimated. Measurements of road verge from a single neighbourhood-scale study in Seattle suggest road verge comprised approximately 4.7% of total land use once total site area was also calculated (Murphy, 2012). In a larger study across 10 census districts in Syracuse, New York, Richards et al. (1984) showed that road verge accounted for 7% of total land use, compared to 9% for parks and 16% for residential gardens. In terms of spatial extent, road verges contributed almost as much as parks.

There are increasing calls to maximise the biodiversity, ecosystem function and human amenity benefits that flow from urban green space (Standish et al., 2013, Haaland and van den Bosch, 2015, Shanahan et al., 2015). Cities tend to occur in biodiversity hotspots (Seto et al., 2012, Threlfall and Kendal, 2018) (Threlfall and Kendal, 2018) and are home to many threatened species (e.g. Ives et al., 2016, Schwartz et al., 2002, Rebelo et al., 2011). Globally, most population growth is occurring in urban areas, 55% of people live in cities (United Nations, 2014) and 3% of earth's surface is now urban (Seto et al., 2010, Zhou et al., 2015). Increasing population densities put greater pressure on existing biodiversity and urban green spaces (Fuller and Gaston, 2009, Haaland and van den Bosch, 2015), so improvements to the ecological function of existing spaces are necessary just to maintain the status quo. Moreover, urban areas need to become more resilient to meet the challenges of climate change (Fünfgeld and McEvoy, 2012).

There are many ways design interventions on road verges can do this. Water sensitive urban design (WSUD), in particular the use of swales and raingardens to improve stormwater retention and infiltration, is a well-accepted approach that is nevertheless still underutilised (Radcliffe, 2018). WSUD on roads is often limited to small interventions rather than being implemented at the watershed scale, though notable exceptions exist, such as research in the Little Stringybark Creek catchment in Melbourne's east (Walsh et al., 2015). Urban forest programmes, such as New York's Million Trees programme (McPhearson, 2011), are larger-scale projects that recognise the many benefits of trees, though urban forest extent continues to decline (Nowak and Greenfield, 2018). In Melbourne all local governments have street tree planting programmes but similar programmes for verge understorey planting are lacking, with only 21% of residential properties in ten local government areas having understorey on the road verge, and that understorey almost entirely the result of resident plantings (Marshall et al., unpublished data). Beyond interventions such as these, broader conceptions of the way the road easement could be reconfigured to maximise the benefits of its green space while recognising the complexity of the trade-offs generally remain unbuilt.

To begin to realise the potential of road easement green space to contribute to this urgent urban greening agenda we need to answer a number of research questions. Firstly, what is the extent of road easement green space's contribution to total urban green space? Secondly, how does it compare to common green space types such as parks and residential gardens? And thirdly, what are the drivers of the extent and distribution of road easement green space?

There are numerous potential drivers of road easement green space and residential garden distribution. Green space provision in cities can vary over time (Haaland and van den Bosch, 2015). Planning controls that regulate elements of the road easement, such as the dimensions of the carriageway (road surface), sidewalks, crossovers (driveways crossing from the property line to the kerb), and the positioning of utilities, have changed over time, suggesting that the distribution of road easement green space might vary with both the date of urban development across a city and the type of road (e.g. major, collector, local) (Van Schagen, 2003, Austroads, 2009, Standards Australia, 2013). Street width, length and age are correlated with canopy cover (Pham et al., 2017). The spatial arrangement of roads, e.g. grid or Radburn-style featuring cul-de-sacs, may also have implications for the extent and distribution of road green space and residential gardens because Radburn-style estates (named after the design of a 1929 estate in Radburn, New Jersey) intentionally minimise road length per residential parcel of land (Stein, 1957), and their different road hierarchies mean different proportions of collector (few in Radburn-style) to local roads. Radburn-style properties have been shown to have less canopy because of the often-irregular shape of their land parcels (Nielsen and Jensen, 2015). Canopy cover is also correlated with street density and neighbourhood age (Lowry et al., 2012), extent of vegetation is correlated with residential parcel size (Lin et al., 2017), and Stone (2004) has shown that the length of residential parcel street frontage is correlated with the proportion of residential land that is garden. Ossola et al. (2019) found residential architectural style to be correlated with extent of both front and back garden canopy. Income, education and other measures of social disadvantage have been associated with the extent of residential garden greenery (Hope et al., 2003, Grove et al., 2006), as has population density (Nesbitt et al., 2019).

To quantify the contribution of road easement green space we mapped its extent and distribution over 47 neighbourhoods in Melbourne, Australia, and compared that to the extent of total urban green space and the extent and distribution of residential garden space. We then investigated the potential relationships between the patterns observed and a range of factors including neighbourhood age, road type, road density and social disadvantage. Only with such data can we begin to craft policy to maximise the benefits flowing from this unique green space type, and to redesign our street and suburbs accordingly.