Nature-Based Solutions for Infrastructure-Induced Urban Residual Spaces: A Systematic Review and Typology-Strategy Alignment Framework

Infrastructure-induced residual spaces the leftover lands beneath metro viaducts, along highway and railway edges, under flyovers, and between built elements constitute a distinct and underexamined category of urban leftover space. Unlike residual land defined by value deficiency or planning status, these spaces originate within transport and mobility systems, which impose specific spatial constraints, environmental degradation, and governance complexities that conventional revitalisation frameworks do not adequately address. This study conducts a systematic literature review of 131 peer-reviewed articles, retrieved using the terms "leftover space" and "residual space", to examine how such spaces have been addressed through nature-based solutions (NBS). Five dominant typologies are identified metro viaduct, highway edge, railway edge, under-flyover, and in-between infrastructure spaces each associated with recurring NBS-oriented revitalisation strategies that reflect varying degrees of ecological intervention, social engagement, and governance involvement. By aligning these typologies with corresponding NBS strategies across policy, implementation, funding, and long-term management, the study develops a preliminary typology–strategy framework to support evidence-based planning and landscape-led regeneration. The findings offer a structured basis for integrating residual urban spaces into sustainable and resilient urban systems.