Thermal comfort in urban open spaces, particularly along pedestrian corridors directly adjacent to heavily trafficked urban arterials, represents a pressing challenge for contemporary sustainable urban planning and landscape architecture. Rapid urbanization, the extensive proliferation of impermeable mineral surfaces, and high anthropogenic heat emissions from dense vehicular movement collectively exacerbate the Urban Heat Island (UHI) phenomenon. This microclimatic degradation leads to severe outdoor thermal stress, diminishes pedestrian walkability, and poses acute environmental health risks. Although urban green infrastructure (UGI) has been widely recognized as an effective passive cooling intervention, comparative microclimatic assessments regarding how distinct spatial configurations and typologies of greenery—such as green roofs, vertical green façades, roadside tree canopies, and pedestrian green bridges—differentially modulate microclimatic conditions across distinct spatial zones (namely, pedestrian pathways versus active vehicular carriageways) remain insufficiently addressed in the literature. To bridge this critical empirical gap, this study conducted a comparative numerical evaluation of diverse green-cover interventions on outdoor microclimatic parameters, with a primary focus on the Physiological Equivalent Temperature (PET) index, across two functionally contrasting spatial domains: a pedestrian sidewalk and the median axis of an urban highway corridor.

The study investigated an urban canyon located adjacent to the Navab Expressway in Tehran, an area characterized by semi-arid climatic conditions, dense high-rise residential morphology, and extreme vehicular congestion. Methodologically, this applied investigation employed three-dimensional microclimate numerical modeling via the ENVI-met (v4.4.5) simulation suite, while spatial distributions and extraction of climatic output data were post-processed using the Leonardo tool. To conduct a rigorous sensitivity analysis, twenty-two distinct scenarios were conceptualized, modeled, and evaluated. These comprised the baseline existing condition alongside individual and compound greening strategies implemented across progressive coverage ratios (20%, 40%, and 60%). The simulated typologies encompassed extensive green roofs, vertical living walls (green façades), street-level shade trees, and an innovative green pedestrian overpass bridge. Simulations were executed over a diurnal cycle on the summer solstice (June 21), capturing the most extreme thermal conditions, with high-resolution temporal data extracted and analyzed specifically for 3:00 a.m. (nocturnal baseline) and 3:00 p.m. (peak diurnal solar radiation and thermal stress). The physical and biometeorological parameters—including air temperature, relative humidity, wind velocity, mean radiant temperature ($T_{mrt}$), and PET—were continuously monitored via two specific receptor points: one situated on the pedestrian sidewalk and another positioned at the center median of the expressway.

The quantitative findings revealed pronounced divergences between micro-scale pedestrian zones and macro-scale traffic corridors. On the pedestrian sidewalk, Scenario 20—a synergistic multi-layered configuration combining 20% vertical green façades, 20% green roofs, and a vegetated pedestrian bridge—demonstrated the superior cooling performance, yielding a notable 1.60°C reduction in PET at 3:00 p.m. relative to the baseline condition. Remarkably, this compound vertical-horizontal green infrastructure strategy outperformed conventional ground-level tree planting, largely due to its combined capacity to obstruct incident lateral radiation, diminish reflected longwave radiation from canyon surfaces, and stimulate localized evapotranspiration without impeding pedestrian wind circulation. Conversely, within the high-speed roadway median, Scenario 13—comprising a high-density integration of 60% green façades and 60% green roofs—emerged as the most effective microclimatic intervention, achieving PET mitigations of 1.03°C during the early morning hours and 0.37°C during peak afternoon conditions.

Furthermore, the comparative assessment showed that the isolated deployment of the green pedestrian bridge exerted an almost negligible influence on reducing mean radiant temperature or PET within the roadway median; however, when strategically integrated with vertical greening along the pedestrian edge, it generated beneficial localized aerodynamic shading and thermal relief for pedestrians. In conclusion, this research demonstrates that optimal cooling strategies for micro-scale pedestrian corridors and broad vehicular thoroughfares are not uniform. Urban landscape architects and municipal authorities cannot adopt generalized greening guidelines; rather, climate-responsive retrofitting requires differentiated, multi-scalar design paradigms that holistically balance vertical surface greening, rooftop modifications, and targeted pedestrian infrastructure to optimize public outdoor thermal comfort.