Abstract
With the rapid expansion of urban areas, roadside green infrastructure has become a critical element of sustainable urban planning. While green areas along highways and streets occupy a relatively small share of the urban carbon sink, they provide disproportionate ecological and social benefits, including improved air quality, carbon sequestration, effective stormwater management, heat island mitigation, and support for biodiversity. This study employs a qualitative comparative case study approach, drawing on literature and policy analysis from Kyoto (Japan), Hangzhou (China), and Riyadh (Saudi Arabia), to evaluate how differing climatic and urban contexts shape the design, performance, and challenges of roadside vegetation. The findings highlight that canopy type, planting methodology, and policy frameworks have a strong influence on the effectiveness of RGI, particularly in regulating temperature, controlling pollution, and supporting ecological connectivity. However, the study also identifies common challenges across contexts, such as spatial constraints, high maintenance demands, and vulnerability to climate extremes. Addressing these issues requires integrated strategies that combine ecological design, long-term governance, and context specific policies. The paper concludes by emphasizing that effective roadside greening can enhance urban resilience, provided it is supported by informed planning and sustained management.