Improving outdoor human-thermal environment by optimizing the reflectance of water-retaining pavement through subjective field-based measurements

The role of urban surfaces in climate is recognized worldwide. Cool pavements such as reflective and water-retaining pavements contribute to the mitigation of urban heat. To optimize the cooling performance of a combination of reflective and evaporative characteristics, water-retaining (WR) pavements, with reflectance values of 13% (WR13) and 25% (WR25), were used and compared to a conventional asphalt pavement (AS). In addition to measuring surface and atmospheric temperatures, human-energy balance known as “human-thermal load” was assessed using perceptive responses obtained from participants (n = 78). Experimental insolation conditions were classified as low-, medium-, and high-based on radiation effects. Water-retaining pavements demonstrated significant surface temperature reductions compared to the overall AS; however, air temperature and humidity above the pavement were less sensitive, and they were not significantly different between the two distinct pavements. Under high insolation, local thermal comfort improved and skin temperatures of the lower body reduced. These consequently reduced the temperature perceived in the whole-body and improved the thermal environment on the surface of WR13. These results can be reasonably explained using the human-thermal load. The increase in thermal load from reflected solar radiation is limited below a certain level of insolation. The overall radiation received by WR13 was moderate from both ground-reflected and -sourced infrared radiation sources. The human-thermal load of WR13 was significantly different from that of AS under high insolation. A modest reflective pavement, such as WR13, is anticipated to improve human thermal comfort.