Painting roads a lighter color seems like an easy fix for hot cities, but a yearlong trial in western Sydney shows there is a catch to the fix. According to the Cool Roads Trial, a 2021 study by Western Sydney University researchers Sebastian Pfautsch and Agnieszka Wujeska-Klause, reflective road coatings cooled down the road surface itself, but made the air immediately above the road feel hotter to people standing nearby during daytime hours. The team tracked this trade-off from February 2020 to March 2021, spanning an Australian summer.
A simple coat of paint, a big temperature swing
The trial report states that the project covered almost 14,700 square meters of roads and carparks across Blacktown, Campbelltown and Parramatta. Councils applied CoolSeal, a light gray water-based coating, over the usual dark asphalt, designed to reflect sunlight rather than absorb it as heat. The results supported that theory. On sunny days, coated roads were 5.9°C cooler at the surface than untreated asphalt, the report says. On the hottest days, the difference was as much as 11.5°C. Researchers have taken hundreds of images with handheld and drone-mounted infrared cameras to capture these differences across the trial sites.
Cooler road, hotter person standing on it
Just because the road surface is cooler doesn't mean it's a cooler experience for people walking or standing on it. Researchers used a black globe thermometer, a device to estimate how hot a person feels in a location based on the combined effects of direct heat, reflected heat, and sunlight, to measure this. The device recorded that the radiant heat load on a person standing near the reflective coating was about 2.7°C higher than near ordinary asphalt during sunny midday hours. When sunlight is reflected from the surface and not absorbed, the energy is not wasted, but it bounces back up and hits whatever is around, including the human body.
