**4. Discussion**

As demonstrated by the thirty-five identified benefits from the manuals, green streets need to be understood as a multifaceted and comprehensive street improvement/sustainable development approach. The following figures (Figure 3) present suggested application of green streets reflecting the typologies identified from the previous chapter with the most common typologies found in the constructed green streets such as cell type stormwater planter (2.A-1), cell type natural (2.A-2), median (2.C-1), extended planter (2.B-1), which were found to be popular in the forty-five green street cases when types of green street application to cities' right-of-way was investigated (Table A1).

**Figure 3.** Examples of green street practice [41] (Modified from pp. 601–601): **0**. Green street on slope with planter type cascades; **1**. Green street with wider sidewalk and seat wall; **2**. Green street with wider sidewalk and landing space for street parking; **3**. Green street with outdoor activity area in conjunction with sidewalk; **4**. Green street with wider and leveled stormwater planter, access to the building, and diagonal parking space; **5**. Green street with extended stormwater planter for traffic calming; **6**. Green street with stormwater planter in median for safe crossing.

These examples show ways to apply different types of green streets into various street settings to create a more walkable and safer environment for users by utilizing available spaces within the right-of-way. The examples are not to represent a certain scale but to demonstrate different compositions of various green street elements visually and specific dimensions are to be determined by available spaces, amount of stormwater runoff from catchment areas, users' needs, and more. It should be discussed that design standards in the majority of the manuals seldom include site context objects such as buildings, street furniture, or dimensions of the sidewalk/roadway, along with other factors that support users' activities. In order to have stormwater planters/swales on streets, a certain amount of space including a pedestrian path is required. Moreover, stormwater treatment facilities need to perform infiltration, provide planting beds, and incorporate underdrain pipes if necessary, and most of all, it should allow the stormwater to be collected in the facility with an opening toward catchment areas. Consequently, for green street practices in particular, underground conditions need to be investigated through on-site inspections because they can differ from what is recorded in existing documentation. If space is insufficient for the subgrade portion of a green street project, it cannot be implemented in the first place. Thus, not only the surface areas of a site, but also its underground conditions (i.e., whether it has a shallow groundwater level and bedrock layers or underground utilities) must be taken into account to ensure the success of a green street project.

This paper mostly deals with green street projects in the US and further study can include case study projects from different countries of various cultures and policies to implement green streets, thus expanding the scope of investigation. Moreover, it must be noted that 'green street' is not a universally adopted term around the world to describe sustainable street improvement with stormwater managemen<sup>t</sup> consideration, hence the term needs to be redefined to be more inclusive for the further research. The findings from this paper will contribute to the knowledge database for green street implementation and the developed typology can be utilized as a system to analyze the diverse sample projects with more multifaceted green street approaches (unique aspects) as well as similar approaches (common aspects) of case study projects around the world. Table 6 shows various features of the case studies in the US, Germany, and South Korea as an example for further research. All projects are implemented on residential streets with green street applications that can be categorized with the typology developed in this paper. However, integrated features can be different as shown in the table: The green street in the US has a flush curb to allow the stormwater runoff to flow into the system as well as parking spaces to accommodate the neighbors' needs [42]. The green street project in Germany is simple but practical with reflection poles for vehicular safety and a series of planters implemented along the street for traffic calming effects [43]. The green street project in Korea has a planter type stormwater facility collecting roof stormwater runoff. The stormwater is stored in a tank and pumped automatically to an adjacent planter with a drought detection sensor. This unique automated watering system is operated by solar power. The wall trellises are used for greening the wall for the narrow street [44]. These examples sugges<sup>t</sup> that further research can present diverse features integrated with green street applications, which will help designers and planners to come up with more effective and creative strategies that can be applied to complex and place-sensitive sites. Its focus will be on multifaceted benefits that green streets can provide rather than solely engineering solutions for stormwater runoff from the street.


**Table 6.** An example of further research: Green street projects in the US, Germany, and Korea.

How green streets can be then discussed in the theoretical paradigm of sustainability? Green streets have been implemented primarily to manage stormwater runoff as one of the strategies of green (stormwater) infrastructure in the built environment in the US. The term green street has been "adopted by many cities to refer to streets that emphasize environmental quality in numerous ways, including reducing pavement widths, increasing tree planting, and incorporating stormwater treatment" [45] (p. 83). However, this stormwater managemen<sup>t</sup> function is only one component of green streets. For instance, the City of Seattle's definition of green streets in its 'Right-of-way Improvement Manual' [29] includes considerations such as promoting pedestrian-oriented streets, enhancing open spaces, managing traffic speed, and adapting to individual localities. Stormwater treatment is mentioned in the manual, but only as one of the possible applications of a green street system, along with solar access, the preservation of historic buildings or street features, and better utilization of the topographical conditions of the site. The other manuals and reports on green streets and green infrastructure that were discussed earlier also describe multifunctional applications of green streets in a community, primarily including their ability to deal with stormwater. Considering this fact, studies of definitions and multiple potential benefits indicate that green streets need to be understood as satisfying a number of diverse strategic characteristics, which can be categorized by three incremental scales in terms of their developmental paradigm: A strategy for stormwater treatment (targeted narrower development strategy), a strategy for green infrastructure (integrated development strategy), and a strategy for sustainable development (inclusive optimal development strategy). Along with the characteristics presented by green streets, it is critical that green street functions as a street need to be

understood in the context of street systems. The following subsections include discussions on these two views.

First, as observed in the previous sections, stormwater treatment function is one of the critical elements that green streets provide. As a component of stormwater treatment systems, green streets are usually integrated with stormwater best managemen<sup>t</sup> practices (BMPs), such as storm tree planters, bioswales, and permeable paving. BMPs are effective treatments for the improvement of stormwater quantity and quality that utilize "natural drainage mechanisms" with "the infiltration and storage properties of semi-natural features" [46] (p. 17). The Virginia Runoff Reduction Method lists the following possible BMPs [47]:


In the US cities, extensive tracts of natural lands have been converted into impervious surfaces due to urban development, and these prevent stormwater runoff from infiltrating into the ground [48]. Without some consideration of where and how runoff flows, problems such as polluted water, erosion, and flooding will eventually arise [49]. Thus, green streets can provide effective networks for stormwater runoff conveyance and release, especially when implemented in environmentally sensitive areas with those issues.

Second, green streets pursue the objectives of the green infrastructure approach—which recognizes the larger scope of environmental issues in addition to BMPs when treating stormwater runoff. Boyle et al. described green infrastructure as "natural and engineered ecological systems which integrate with the built environment to provide the widest possible range of ecological, community and infrastructure services" [50] (p. 5). Similarly, Benedict and McMahon stated that green infrastructure is "an interconnected network of natural areas and other open spaces that conserves natural ecosystem values and functions, sustains clean air and water, and provides a wide array of benefits to people and wildlife. ... [It] is the ecological framework for environmental, social, and economic health" [51] (p. 1). Others have suggested that when green streets are considered for treating stormwater, a watershed approach needs to be adopted, without regard for political boundaries. On-site treatment to enable effective protection of the water quality and protect downstream sites is important, and the plans should consider human health and property as well as ecological preservation [52,53].

The American Society of Landscape Architects has described green infrastructure as "a conceptual framework for understanding the 'valuable services nature provides the human environment'" [54]. The ASLA also characterizes green infrastructure on three scales: (1) At the national or regional level, with a consideration of networks of parks and wildlife corridors; (2) at the urban level, with a consideration of parks and urban forestry as ecologically central hubs; and (3) at the level of buildings, with a consideration of smaller applications such as green roofs and green walls. The ASLA also cites multiple benefits of green infrastructure, such as reducing energy use, improving water and air quality, decreasing solar heat within a city, providing a wildlife habitat, controlling floods, reducing the cost of stormwater treatment facilities and preventing erosion [54]. As one constituent component of green infrastructure recommended by the US EPA, green streets are considered to provide those multiple benefits rather than focusing on only stormwater treatment. Green streets should be applied as a network, reflecting local needs. They play an important role in the larger landscape as a way of improving and protecting the built and natural environments to which the larger ecological conception is applied.

Lastly, green streets can be discussed in the context of sustainable development. Sustainable development follows a holistic approach that promotes balance between human settlements and natural environments in planning processes. The United Nations defined sustainable development in their report, Our Common Future, as a development that "meet[s] the needs of the present

without compromising the ability of future generations to meet their own needs" [55] (p. 37). Similarly, the Department for Environment, Transport and the Regions (DETR) in the UK defined it as follows: "sustainability is about ensuring a better quality of life for everyone—now and for generations to come" [56]. These definitions do not mention a specific scope for the concept of sustainable development, although it can be inferred that sustainable development is extensively interconnected with phenomena on the earth that are caused by humans and nature. Other descriptions of sustainable development note that it is "elusive ... [but] important and does deserve attention. ... the core of the idea of sustainability is the concept that current decisions should not damage prospects for maintaining or improving living standards in the future" [57,58] (p. 716). Ritchie and Thomas have described sustainability in urban design as planning a city with more consideration for people. In this sense, "creating pedestrian, cyclists and public transport is a key aspect of sustainable development" [59] (p. 3), a clear contrast to existing built environments that are overwhelmingly automobile oriented. Ritchie and Thomas have described two more aspects of urban sustainability: making a city greener for biodiversity as well as for the well-being of humans and creating a more delightful environment to keep people "secure and happy" [59] (p.3). The specific scope of sustainable elements can be described as follows: "1) To protect natural resources, 2) to use resources e fficiently, 3) to strengthen the sense of community, and 4) to consider the regional context" [51]. Moreover, sustainable development is frequently recognized as development that is "ecologically sustainable or environmentally sound" [60,61] (p. 608). Consequently, successful achievement and balance among three aspects of sustainability—economy, environment, and society [55]—in the larger scope of the environment can be promoted by adopting a sustainable development approach, which green streets potentially and optimally aim for.

In conclusion, sustainable urban development does not always include a green infrastructure approach, and a green infrastructure approach does not always include stormwater managemen<sup>t</sup> considerations (Figure 4). However, the planning and implementation of stormwater managemen<sup>t</sup> and a green infrastructure do need to be discussed in the context of sustainability, where sustainability means more holistic and renewable strategies for improving and protecting a built and natural environment. As Odum has stated, humans have forgotten that ultimately, they depend for everything on nature, and they are likely to continue to forget this "as long as life support services [from nature] are considered free" [62] (p. 3). Therefore, pursuing sustainable development does not just involve taking into account the well-being of humans or the reclamation of one part of a river corridor but is more about realizing how the elements of an environment are integrated on a larger scale. When we are planning and developing new projects, we need to realize what kind of impact we ourselves, as parts of the environment, will have on the whole environmental system. In this sense, sustainability does not always mean green, but green tries to achieve sustainability.

**Figure 4.** Green street in three development paradigms.

As an element of green infrastructure, an ideal green street project seems to be one that is designed using a complex strategy for pursuing sustainability. It can "act as a synthesis of a number of other areas of planning (greenways, green spaces, high-density planning) to promote a coherent discipline for future development" [55] (p. 24). Therefore, green streets should be designed differently from a conventional stormwater treatment system, which is solely designed to target a very specific goal such as reducing stormwater runoff or preventing local flooding. The multiple objectives of sustainable development are the ultimate goal for green streets in relation to a given site. This means that green streets need to emphasize a network system approach that can successfully integrate multifunctional disciplines to promote their influence throughout the site.
