The effect of urban greening on pedestrian’s thermal comfort and walking behaviour

¹ Department of Civil and Environmental Engineering, Faculty of Construction and Environment, The Hong Kong Polytechnic University, Hong Kong.
² Department of Building, School of Design and Environment, National University of Singapore, Singapore.

^* Corresponding author: yuhong.wang@polyu.edu.hk

Abstract

The urban heat island (UHI) phenomenon is becoming increasingly severe due to unprecedented urbanization and global warming paces. The increasing heat stress threatens the environment, the health of residents, and also the city the walkability. However, greenspace can generate a cooling effect to mitigate the UHI. To gain a better understanding of how urban greening contributes to the optimization of outdoor thermal comfort and the improvement of city walkability, this study investigates the relationships between outdoor thermal environment, the thermal comfort of pedestrians, and their traveling behaviours. Thermal environment was simulated using a microclimatic computational fluid dynamic (CFD) model – ENVI-met. Travel behaviour of pedestrians was simulated through agent-based modelling (ABM). A total of 337 pedestrians were monitored and interviewed across several outdoor sites with different urban morphologies in Hong Kong, along with the simultaneous collection of site-specific climatic data. Based on the data, relationships between outdoor thermal conditions, human thermal perceptions, and walking speeds were analysed exploratively and quantitatively. It is found that the walking speed of pedestrians is notably reduced with increased thermal stress levels. The walking speed can be well predicted by a polynomial regression model (R²=0.719), artificial neural network (ANN) models (R²=0.907), and a deep neural network (DNN) model (R²=0.931). Street trees can improve outdoor thermal comfort effectively (a maximum reduction of the mean radiant temperature at 4.23 °C and a maximum reduction of the universal thermal index at 0.88 °C). Simulation results of ABM demonstrate that street trees can cause a reduction in perceived travel time (PTT) of up to 3 s per 100 m. The research findings are expected to mitigate urban warming and constitute thermally comfortable and walkable outdoor environments.