Phoenix is one of the hottest cities in the United States, so in 2020 the city tried a new way to cool things down. Rather than plain black asphalt, the city painted some roads with a light-colored, reflective coating, on the theory that a lighter road would keep cooler. A year-long study was performed by Arizona State University (ASU) to test this, and the Cool Pavement Pilot Program report summarizes the results.

What Phoenix actually did

The City of Phoenix Street Transportation Department teamed up with ASU’s Urban Climate Research Center. In the period between July 15, 2020 and July 14, 2021, they used the coating for roads in various neighborhoods and then compared them with uncoated asphalt in the surrounding areas. At the surface level, the coating did its job. The ASU report says treated roads were 2.4 degrees Fahrenheit cooler at sunrise, 12 degrees cooler at noon, and 10.5 degrees cooler in the afternoon compared to conventional, aged asphalt. Even the ground beneath the surface was an average of 4.8°F cooler, which matters in a city where roads can get hot enough to burn skin.

The catch nobody expected

A cooler road doesn’t necessarily mean a cooler walk. The reflective coating reflects more sunlight than it absorbs, which helps keep the surface cooler. However, the reflected rays are then directed back at the pedestrians walking close to the surface. The research found that mean radiant temperature, a measure of the thermal radiation absorbed by the body outdoors, rose by an average of 5.5°F on the coated streets. The finding is consistent with other research. In a separate study titled ‘Solar reflective pavements: A policy panacea to heat mitigation?’ published in Environmental Research Letters, ASU researcher Ariane Middel and colleagues tested a similar coating in two Los Angeles neighborhoods. It noted surface temperatures dropped 4°F to 6°F, but mean radiant temperature over the pavement rose about 4°C (7.2°F) at midday, while air temperature fell only slightly.

The coating also wears off over time. It was brightest just after it was applied and reflected about 33 to 38 percent of sun radiation, but after ten months of exposure, this figure was down to 19 to 30 percent, due to accumulated dirt and general wear. A typical asphalt surface reflects about 12 percent of sunlight. Even a faded coating is better than plain asphalt, but the margin is smaller.

A longer-lasting second version, and a bigger-picture studyBased on these results, Phoenix and ASU tested a new version of the pavement coating in seven additional neighborhoods from May 31 to June 6, 2022, and released their findings in October 2024. Cool pavement technology, according to this phase of the project, is capable of reducing summertime daytime temperatures of pavements by as much as 12°F compared to old pavements, saving money on road maintenance, and having some effect on air temperature.

In a subsequent review of Phoenix's early data, published in Nature Communications, researchers looked at the effect of reflective pavement on surface, air, and radiant heat combined, and found that reflective pavement technology does not work everywhere; it works best on open residential roads and parking lots with no shade and low pedestrian traffic, and should not be applied in places where people congregate at noon, such as playgrounds and plazas.

Cities are still figuring this out

Phoenix is not alone. Other Sun Belt cities, including San Antonio, are also testing cool pavement, according to a report by the American Society of Civil Engineers. Experts quoted in the report admit that cities are still unsure whether it actually improves pedestrian comfort. Some have put programs on hold to study the trade-offs first. The ASCE report says San Antonio tested cool pavement on one street in 2021, then expanded to selected streets in 10 neighborhoods after the city council approved $1 million in 2023. Researchers found the most effective coating cut afternoon surface temperature by an average of 3.58 degrees, with the gap widening to 18 degrees against freshly paved asphalt, a darker, hotter surface than the aged asphalt used as the baseline in Phoenix's own study.

The takeaway

Reflective coatings and cool roofs are already included in several heat action plans around the world, and hot cities around the globe are watching Phoenix closely. Phoenix’s experience is a good reminder before any city paints its roads white or silver: cooling a road surface and cooling the person walking on it are two different problems, and fixing one doesn’t automatically fix the other. The cool pavement technique is neither a gimmick nor a failure. It reduces the surface and sub-surface temperatures, which helps the road last longer. However, if the goal is pedestrian comfort, cities may need to consider shade, tree cover, and where these coatings are used, not just the color of the coating.