**1. Introduction**

Urban sustainability in the US is at risk as the ecosystems of cities have been impacted by increasing population density in urban areas [1], further challenged by climate change such as extreme weather conditions (e.g., urban flooding, droughts) [2]. Rapid development in cities with high population density results in the increase of impervious surfaces (e.g., paved surfaces, building footprints) that a ffect urban ecosystem with excessive stormwater, accumulation of pollution in streams [3,4], and temperature increase (e.g., urban heat island) [5]. Extreme weather conditions such as urban flooding causes massive destruction on infrastructure, economics, and human lives in a ffected regions [6] while heat waves make people ill and die as exposed to extreme temperature for a longer period of time [7]. While impacts of extreme weather are aggravated by the excessive imperviousness leading to extreme flooding events [8] and heat waves [7], the consequences of urbanization and climate change ultimately threaten human well-being and our environment.

In regard to the mitigation of such conditions in urban cities, green infrastructure can provide an alternative solution by treating and reducing impervious surfaces to amend and improve already challenged urban ecosystems by using natural materials such as soils and plants. Adopting green infrastructure can promote sustainable urban ecosystems by increasing vegetation with greater biodiversity, improving well-being by promoting more green space [9], and climate resiliency by treating stormwater [10] and urban heat island e ffects [11].

However, despite its affordable and multifunctional benefits that can prove to be very effective solutions [12], its application is sometimes discouraged due to lack of comprehensive guidance [13] and "lack of understanding and knowledge of what green infrastructure is and the benefits it provides" [14] (p. 2). Thus, this paper focuses on green streets (also called green alleys), one of the eleven elements of green infrastructure as recognized by the US Environmental Protection Agency (EPA) (apart from downspout disconnection, rainwater harvesting, rain gardens, planter boxes, bioswales, permeable pavements, green parking, green roofs, urban tree canopy, and land conservation). In addition to the potential benefits that municipalities and institutions intend to provide in the US, this paper focuses on examples in the US to fill the gap in the scholarly literature lacking topics primarily dedicated to green streets in the US. Given the fact that the sum of the US public roads and streets is about 4,146,410 miles long [15], almost half of urban areas are dedicated to streets, highways, and parking areas [16], and roads are primary reasons related to issues of impervious surfaces [17], studying and promoting green streets can be an effective solution for issues related to impervious surfaces among other elements of green infrastructure. Street greening projects without stormwater managemen<sup>t</sup> consideration were not treated in this paper. The goal of this paper is to provide a better understanding about green streets of comparatively recent development approaches for those concerned with urban ecosystems with interests in implementing such projects and also for members of the public who have little knowledge about what green streets are and how they can benefit communities [18]. The long-term goal is to help promote the acceptance and successful adoption of the green streets by probing its potential as a sustainable strategy in achieving livable and healthy communities in urban cities. This study investigates various definitions, objectives, and types of stormwater treatment facility designs as outlined mainly in selected manuals. The author then discusses future green street studies by taking into account case study projects and their applications from other countries while examining the roles of green streets in the context of sustainable development paradigms.

### *1.1. Definition of a Green Street*

Green streets in the US are known as on-site solutions installed primarily in the public right of way (public street/passage) to treat polluted stormwater runoff generated from impervious surfaces and conveyed to local waterways without filtration [16]. The implementation of green streets on the public right of way is often times easier than the implementation on private streets due to ownership issues. It mainly reduces impervious surfaces by adding more vegetation to increase permeable areas for treating stormwater runoff, which is one of the key design components of green streets not featured in conventional street designs. As a stormwater treatment measures, the construction of green street impacts appearances of streets and technical applications in different ways, depending on the types of pollutants, available space, land uses, and community needs that are involved.

Green street manuals and websites of various cities and agencies implementing this approach are used to arrive at a common understanding of how green streets are defined (Table 1). Aspects of green streets were generally agreed on: (1) They are implemented within the right of way, (2) they are a means of treating stormwater runoff, and (3) the adopted stormwater treatment process includes vegetation and soil beds. In sum, the grea<sup>t</sup> majority of the definitions depict a green street as a sustainable stormwater treatment practice that is mainly implemented in the right-of-way and includes the utilization of natural materials for this purpose. As an example, Portland, OR is one of pioneering cities that adopted green streets, with the first green street project being implemented as early as 2003 [19]. The city approved the green street approach in public and private development in 2007. The working definitions of this approach used by Portland Environmental Services and Metro (Portland Metropolitan Area) primarily emphasize stormwater treatment and watershed health. Most other cities in Table 1 which are now adopting projects of this nature also appear to view them as a means of stormwater managemen<sup>t</sup> and to be working from a definition of green streets that is similar to Portland's. While the definitions primarily emphasize stormwater managemen<sup>t</sup> functions, there is

some variability in the objectives of green street projects across the country, depending on the goals of the individual cities to be discussed in a later section.


**Table 1.** Definition of green streets.

### *1.2. Green Street versus Street*

The definition of a street according to Cambridge Dictionaries Online is "a road in a city or town, usually with buildings along one or both sides." Conventional streets are seldom integrated with stormwater treatment systems or connected to open spaces; they are automobile-oriented thoroughfares with a high proportion of impervious surfaces and decreased pedestrian safety [28], and they utilize prescribed street standards that limit the type of multifunctional adaptation considering the pedestrians' use of the street.

In contrast, green streets can be distinguished from typical streets in a number of respects (Table 2). As noted by the EPA, green streets incorporate integral stormwater treatment systems that are designed to improve the water quality and reduce the quantity of runoff [22]. In most cases, they are also connected with open spaces [29] and intended to be pedestrian-oriented for the purpose of supporting a safer and healthier community [20,23]. Green streets are implemented with less impervious surfaces than conventional streets because they feature narrower roads and wider planting beds (which create more available permeable surfaces); green streets also provide a safer walking environment for local residents [24]. In addition, green streets potentially allow flexibility for planners and designers to come up with designs that meet local needs. For instance, integrated stormwater treatment facilities can be of different sizes, and the plants used can be varied to suit local climate conditions, as suggested in numerous green street manuals.

Green streets, however, also need to retain their traditional function as streets because, although they may have design applications for a better community, they are implemented in the right of way. Streets are one of the key elements that physically comprise a city and can be exciting places "where people walk, shop, meet, and generally engage in the diverse array of social and recreational activities that, for many, are what makes urban living enjoyable" [30] (p. 283). Streets are also important elements that connect one point to another, substantially forming and shaping a city. Streets run between private and public spaces, conveying both pedestrians and vehicles from one place to another. Thus, the benefits of integrated elements of streets are distributed throughout the adjacent communities and to the people who use and live on the streets. Therefore, streets can help form the character of a city influencing its culture and history. They, along with building footprints, comprise one of the largest areas of impervious surfaces in an urban area, and impervious surfaces are a useful measure for other features such as the water quality in local streams in relation to stormwater runoff generated from them [31]. Moreover, the flow of water is affected by the patterns of streets [26]. Thus, important roles of streets need to be considered with the adoption of green streets including proper facilities (stormwater management, street furniture, vegetation strips, and other recommended facilities and design features) implemented within the context of the larger street network: Applying the effective and creative approach of green streets should be an useful strategy for resolving urban runoff problems generated from impervious surfaces in a more sustainable way as providing multiple benefits.
