Scientists tested reflective pavement through an Australian summer, and the road surface stayed cooler, but midday radiant heat increased above it

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.

This is not a unique finding. A separate 2020 study titled ‘Solar reflective pavements—A policy panacea to heat mitigation?’ in Environmental Research Letters discovered the same trend when studying the effects of reflective coatings applied to the streets of Los Angeles. The study found that the mean radiant temperature, or the amount of heat received by a human body, increased by 4°C at noon, while the pavement surface itself remained 4–6°C cooler compared to regular asphalt pavement.

Air temperature barely moved

Another interesting finding of the Western Sydney trial was that the coating had little effect on the temperature of the surrounding air, as opposed to the radiant heat load discussed above. Continuous temperature sensors placed along the trial streets did not show a consistent cooling effect on ambient air temperature during daytime. The trial report attributes some slightly warmer air readings at certain sites after coating to heat bouncing off the surface and warming the air just above it, though the effect was not consistent across sites. A few sites showed a small nighttime cooling, generally on the order of 0.3°C, but that effect also wasn't consistent across sites.

Why Australian cities are still cautious

Cool roads haven't been widely adopted across Australia, ABC News reports, despite roughly a decade of trial programs in various cities. It reports that a pilot project conducted in Adelaide involving the application of CoolSeal coating was able to lower the road temperature by up to 8.65°C in daytime and 4.2°C at night. However, that report also pointed out that reflective surfaces tend to make pedestrians standing on them feel hotter, hence the slow rollout of the technology in major streets.

What this means going forward

The researchers involved in the Western Sydney project do not deny that reflective surfaces are effective. The trial documented a measurable reduction in surface heat buildup, which is useful for urban infrastructure. However, the researchers recommend supplementing or replacing it in pedestrian areas with trees, shade structures and porous pavements that allow evaporation. Reflective pavement redistributes heat rather than eliminating it, and where that heat ends up matters as much as the color of the road itself.