Microclimate Comfort of Elevated Bridge Underpass Spaces Based on Sound and Thermal Environments: A Case Study of Fuzhou and Xiamen, Fujian Province

In the context of urban renewal, spaces beneath elevated bridges are gradually becoming important public activity areas, with their microclimate and landscape quality influenced by multiple factors including bridge structure, vegetation configuration, and traffic conditions. This study focuses on 12 typical elevated bridge underpass spaces in Fuzhou and Xiamen, two hot–humid subtropical cities in China, and integrates field environmental monitoring, questionnaire surveys, spatial landscape metrics, and statistical analysis to explore how plant landscape factors are significantly associated with thermal and acoustic conditions and overall comfort levels. Results show the following: (1) Overall comfort refers to the overall subjective evaluation of the elevated bridge underpass environment under combined thermal and acoustic conditions. Thermal and acoustic comfort jointly explained 71.3% of its variance (R² = 0.713), with thermal comfort contributing more strongly. (2) Bridge orientation was associated with differences in shading conditions and microclimatic characteristics, with east–west-oriented elevated bridge underpass spaces generally offering better overall comfort. (3) Green coverage rate, green buffer width, distance from roads, and hard paving proportion were identified as key landscape factors associated with overall comfort. Controlled regression showed significant associations, with green coverage rate showing the strongest positive association with OCV (β = 0.612), whereas hard paving proportion showed the strongest negative association (β = −0.628); the models explained 66.8–68.4% of OCV variance. (4) Appropriate plant landscape configurations were associated with more favorable microclimatic conditions under bridges. Within the studied sites, higher comfort was associated with approximately 40–55% green coverage, 8–15 m green-buffer width, a road distance above 60 m, and a hard paving ratio below 50%. These values should be regarded as context-specific design references derived from the observed regression patterns rather than universal optimal thresholds. This study identifies associations among plant landscape configuration, microclimate, and environmental comfort in elevated bridge underpass spaces based on field evidence from Fuzhou and Xiamen, providing context-specific references for landscape renewal in similar hot–humid subtropical settings.