The Experiment
Phoenix, one of the hottest cities in the United States, launched an experiment in 2020 to combat extreme heat by coating 36 miles of neighbourhood roads and a public parking lot with CoolSeal, a grey, water-based asphalt treatment. The City of Phoenix Street Transportation Department partnered with Arizona State University's Urban Climate Research Center to study the effects over a year, from July 15, 2020 to July 14, 2021. The results were summarized in the Cool Pavement Pilot Program report.
The theory was straightforward: a lighter-colored, reflective road would absorb less sunlight and stay cooler than traditional dark asphalt, which can reach temperatures of up to 180°F during the hottest summer periods.
The Cooling Effect
The coating proved effective at lowering surface temperatures. According to the ASU report, 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. City engineers noted that this could help roads last longer and reduce maintenance costs.
However, the cooled surface came with a trade-off for people on foot.
The Pedestrian Problem
ASU researchers used a mobile weather station called 'MaRTy' to measure mean radiant temperature, which quantifies the heat radiation absorbed by the human body outdoors. They found that on coated streets, mean radiant temperature rose by an average of 5.5°F during the hottest part of the day. This increase happened because the reflective coating bounces sunlight sideways and upwards rather than absorbing it, directing heat toward pedestrians.
ASU scientist Ariane Middel, who was involved in the project, emphasized the importance of this metric: "Perhaps the most meaningful measurement is the mean radiant temperature." Similar findings emerged from a separate study by Middel and colleagues, published in Environmental Research Letters, which tested a similar coating in two Los Angeles neighborhoods. There, 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 Los Angeles results confirm that this is not a localized anomaly but a broader pattern. Jennifer Vanos, associate professor in ASU's School of Sustainability, explained the real-world implications: "If you're standing over one of these surfaces on a hot day, you're still going to be hot if you're not in the shade... it's not going to be just paint all the streets white. You're going to have to really keep thinking about shade."
Durability and Reflective Efficiency
The coating's reflective properties also degraded over time. Just after application, it reflected about 33 to 38 percent of sun radiation, but after ten months this dropped to 19 to 30 percent due to dirt and wear. Traditional asphalt reflects only about 12 percent of sunlight. This decline suggests that the cooling effect may diminish unless coatings are maintained or reapplied.
What Cities Can Learn
The Phoenix experience offers lessons for other cities exploring cool pavement technologies. A review published in Nature Communications found that reflective pavement 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. This aligns with the Phoenix findings: the coating's benefit depends on where and when it is used.
San Antonio is also testing cool pavement, according to a report by the American Society of Civil Engineers. The city tested the coating on one street in 2021 and, after city council approved $1 million in 2023, expanded to selected streets in 10 neighborhoods. The most effective coating in San Antonio cut afternoon surface temperature by an average of 3.58 degrees, with the gap widening to 18 degrees against freshly paved asphalt.
Phoenix and ASU did not stop with the first trial. They tested a new version of the coating in seven additional neighborhoods from May 31 to June 6, 2022, with findings released in October 2024. Later phases of the programme tested newer versions, including CoolSeal 2.0, and the city expanded its Cool Corridors Program.
The technology is now part of broader heat mitigation strategies: reflective coatings and cool roofs are included in several heat action plans around the world. Yet, as the Phoenix data shows, it is neither a gimmick nor a failure. It reduces surface and sub-surface temperatures, helping roads last longer, but it does not automatically make streets more comfortable for pedestrians. For that, cities may need to pair cool pavement with shade, tree cover, and careful choices about where these coatings are applied. Open residential streets and parking areas with little natural shade may need additional measures, particularly trees and shade structures, to ensure that efforts to cool the pavement do not increase heat exposure for pedestrians.