**1. Introduction**

In scholarly literature, greenways have been defined as "networks of land containing linear elements that are planned, designed and managed for multiple purposes including ecological, recreational, cultural, aesthetic, or other purposes compatible with the concept of sustainable land use" [1]. Early use of the "greenway" term can be traced to Elenor Smith Morris' publication of "New urban design concepts: greenways and movement structure: the Philadelphia plan" in *Architect's Yearbook* [2], and William H. Whyte's *The Last Landscape* [3], which describes greenways as critical linkages and connectors in a hierarchy of urban green spaces. The idea gained further scholarly traction with *Greenways for America* [4] and *Greenways: A Guide to Planning, Design, and Development* [5], followed by two Special Issues of a peer-reviewed journal, *Landscape and Urban Planning*, dedicated to this topic [6–8].

In practice, however, greenway precedents include tree planting along roads and canals, dating back 2000 years in China [9]; landscape corridors dating to ancient Rome; planted boulevards in 18th century European cities; and the 19th century parkways and park systems of U.S. cities [10,11]. And today, there is substantial interest in greenway planning and design. This is especially true in urban areas due to the growing concentration of people in cities [12], and the limited amount of available space in increasingly built up settlements. Greenways may also be of contemporary interest in so-called "legacy" or "shrinking" cities that are characterized by a diminishing population and

increasing swaths of vacant land [13,14]. As such, urban greenways may be a form of 21st century landscape planning and design that has the potential to address the challenges and opportunities of both expanding and contracting cities, as noted at the 6th Fábos Conference on Landscape and Greenway Planning at which a preliminary version of this study was presented [15].

In this paper, we treat urban greenways as a related but distinct subset of greenways writ large. We believe this is justified for reasons pertaining to pre-existing condition, location, and extent. Regarding pre-existing condition, greenways are often characterized as undeveloped and environmentally sensitive corridors to be conserved in advance of urbanization [1,5]. This highlights the importance of greenways as a sustainable planning strategy to contain or shape urban expansion, reduce land fragmentation, and maintain "landscape integrity" [16], drawing upon scholarship in landscape ecology [17] and subsequent literature on green infrastructure [18], where greenways are critical links/corridors that connect hubs/patches of natural lands to support biotic and abiotic ecological processes. Stated another way: greenways are critical elements of "nature's...pre-existent...super infrastructure" [19].

Pre-existing condition also relates to location. The Oxford English Dictionary, for example, defines a greenway as "a grassy path or way; a piece of undeveloped land *near* an urban area, set aside for recreational use or environmental conservation" [20]. Regarding extent, greenways are often conceived as a regional, state, national, international or even continental network that can include but generally transcends urban areas [1,11,21–23]. One of the most prominent greenways in the United States, for example, is the Appalachian Trail, which extends some 2190 miles along the eastern mountain chain from which it takes its name; and the experience it is intended to provide is essentially an escape from the urban condition [24].

This is not to suggest that urban areas are not conducive to greenways. Indeed, noteworthy sites in landscape planning history include urban greenways [11]; greenways can constitute a "living network" that provides "people with access to open spaces close to where they live...and link together rural and urban spaces" [25]. Yet, urban areas differ from non-urban areas in important ways, including biophysical flows, sociopolitical dynamics, and formal structure [26–30]. Moreover, in much the same way that urban parks can differ from non-urban parks [31,32], greenways located in highly urbanized areas contend with conditions that can be quite different from non-urban areas [33,34]. Thus, it behooves landscape planners and designers—and associated scholarship—to understand the unique traits of urban greenways to meet the needs of current and future cities.

Towards that goal, this paper addresses the following question: What are the traits that distinguish urban greenway scholarship and practice as an evolving form of landscape planning and design? With this in mind, we address three objectives: (1) illuminate the ways that urban greenways may be a distinct subset of greenways writ large; (2) systematically review scholarly literature on urban greenways; (3) develop a typology and definition of urban greenways. To the best of our knowledge, there has been no previous literature review on greenways in an urban context. Literature reviews are foundational for advancing state-of-the-art understanding of a topic [35,36], and for doing substantive and thorough research [37]. We address this gap and the aforementioned topics by reporting on a systematic literature review of urban greenways and development of an associated typology. Although urban greenways tend to be designed by landscape architects and urban planners, they are also domains of research for a range of fields, including ecology, geography, sociology, wildlife conservation, economics, human health, and others. A systematic review, typology, and definition may, thus, facilitate scholarship on urban greenways across a range of disciplines.
