Solar reflective pavements – a policy panacea to heat mitigation?

Cities worldwide are piloting the use of solar reflective coatings on roads to mitigate the Urban Heat Island (UHI). Solar reflective pavement has been praised as simple, low-cost solution with a demonstrated ability to reduce surface temperature and, potentially, energy use. Less well understood is the effect of solar reflective coatings on radiant heat, which influences human thermal exposure and comfort. We present the first biometeorological observations of solar reflective coating to investigate its thermal performance from a pedestrian perspective. Hourly transects were conducted in two Los Angeles neighborhoods with MaRTy, a mobile platform that measures air, surface, and mean radiant temperature. Transects were performed on July 30, 2019, a typical summer day with low wind speeds and maximum air temperature of 31°C. The surface temperature of the coated asphalt concrete was 4°C to 6°C lower than that of the regular asphalt concrete, but coated surfaces reflected 118 Wm-2 more shortwave radiation on average and up to 168 Wm-2 more at noon. In the evening, MaRTy observed 20 to 30 Wm-2 of added reflected shortwave radiation on sidewalks next to the reflective pavement. Mean radiant temperature over reflective pavement was 4°C higher during midday. Although air temperature was reduced by 0.5°C in the afternoon, after-sunset cooling was negligible. Findings illustrate the benefits and disadvantages of reflective pavement with respect to various thermal performance metrics. Cities should weigh the tradeoffs of UHI mitigation, thermal exposure, implementation and maintenance costs, lifecycle, and other competing priorities in the context of space use.