From Bare Pavement to Shaded Corridors: Quantifying Thermal Comfort Gains and Optimal Tree Density on Jeddah’s Walkways

Urban greening is frequently advocated as a method to alleviate pedestrian heat stress in hot-arid and hot-humid coastal cities; however, planting schemes are frequently designed according to quantitative objectives rather than performance-based thermal outcomes. The thermal comfort gains achieved by the Al-Yamamah walkway in Jeddah, Saudi Arabia, as a result of the corridor’s vegetative transformation between 2008 and 2026 are quantified in this study, which also explores, through a single evolutionary optimization run, whether an alternative tree arrangement lowers mean UTCI in a separate simulation framework/The Universal Thermal Climate Index (UTCI) was employed to reconstruct and evaluate historical and contemporary tree configurations during peak summer conditions (12:00–16:00, July) using Autodesk Forma. The ground area under very strong heat stress was reduced from 99% to 92% as a result of the nearly three-fold increase in in-walkway trees (from 10 to 25) and an 86% increase in canopy diameter (from 3.5 m to 6.5 m). In Rhino/Grasshopper, a subsequent parametric optimization was conducted using the Galapagos evolutionary solver to maintain a fixed canopy diameter of 6 m. This single optimization run identified a 46-tree configuration as the best solution found in that run, with a mean UTCI of 40.06 °C across the same peak window. Because tree count (25 in 2026 versus 46) and tree placement were varied together, and because this value comes from a different tool and metric than the Forma ground-area percentages reported for 2008 and 2026, it is presented as a separate optimization result and not as a like-for-like improvement over the 2026 condition. The empirical 2008–2026 comparison suggests that comfort outcomes are more strongly associated with canopy maturation than with tree count alone, although the study design does not allow these geometric factors to be statistically isolated from one another. Additionally, the optimization run indicates that layouts with a lower simulated mean UTCI can be found for a fixed 6 m canopy specification, although the separate contributions of tree count and tree placement were not isolated. A transferable, performance-validated framework for aligning Vision 2030 greening investments with measurable pedestrian thermal benefits in Jeddah and analogous coastal cities is provided by the two-stage workflow, which consists of empirical before/after assessment and evolutionary placement optimization.