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  • Writer's pictureOlivia Diem

Cool Pavement: A year later. Just how cool is it?

In 2020, the City of Phoenix and Arizona State University teamed up to introduce the Cool Pavement Pilot Program, the solution to Phoenix’s urban heat island effect. This year, on September 14 the scientific test results revealed that the reflective pavement had considerably lower surface temperatures than traditional pavement.

The city of Phoenix is one of the many cities around the world that are experiencing rising temperatures due to our changing climate.

Summers in Phoenix can get close to 120 degrees Fahrenheit thanks to the urban heat island effect, a phenomenon where temperatures are significantly warmer in urban areas than surrounding rural areas due to infrastructure and human activities.

“Pavements can cover about 30-40 percent of the urbanized area and contribute to the high temperatures,” Special Projects Administrator for the Street Maintenance Division Rubben Lolly said.

Cool pavement is a water-based coating that is light in color and reflects higher amounts of sunlight and absorbs less heat than traditional pavement. The goal is to reduce the temperatures that roads give off both during the day and at night.

The coating was applied to select portions of eight neighborhoods in each city council district in 2020 to begin the study.

In just one year, the pavement has been proven to reduce surface temperatures at all times of the day, according to the City of Phoenix Street Transportation Department and Office of Sustainability.

On average the “cool pavement was 12 degrees Fahrenheit to 10.5 degrees Fahrenheit cooler at noon and afternoon hours. At sunrise, it was approximately 2.4 degrees lower and at sunset, it was approximately 4.8 degrees Fahrenheit lower,” Assistant Professor for the School of Sustainability Jennifer Vanos said.

The data used to measure the surface temperature was collected using an infrared radiometer attached to the bottom of a vehicle and air temperature was measured using fine-wire thermocouples.

The goal of the project, while right now focussed on reducing surface temperatures is “ultimately, to improve livability in the city, but also with additional benefits,” Lolly said.

The pavement is already showing ways it can benefit the city environmentally and economically.

The cool pavement is made with asphalt, water, an emulsifying agent (soap), mineral fillers, polymers and recycled material making it a step closer to sustainability.

Mark Hartman, chief of sustainability for the City of Phoenix Office of Sustainability, said the most intriguing thing is “because the surface is so much cooler it actually increases durability and will actually make it more economic than traditional coatings.”

Another test was conducted to test the performance of the cool pavement below its surface, and it averaged a 2.1-degree Fahrenheit difference between the top and bottom of the pavement.

“We expect that that will help prolong the asphalt in addition to the length of the coating itself protecting the asphalt. It’s like sunscreen,” Ryan Stevens, civil engineer III, pavement management engineer, said.

The reduced thermostress the coating provides the pavement should ultimately give the roads a longer life, making the cool pavement more economically friendly in the long run.

The team seems to be very happy with the results so far and looks forward to further testing the longer-term performance as well as assessing the related environmental, health and social benefits the cool pavement can bring.

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