Photo: City of Phoenix

Phoenix cool pavements pilot delivers promising results

20 September 2021

by Sarah Wray

The City of Phoenix has released the results of the first year of its Cool Pavement Pilot Program.

The findings so far show that cool pavements had lower surface temperatures at all times of the day versus traditional asphalt.

The project, run by the City of Phoenix Street Transportation Department and Office of Sustainability in partnership with Arizona State University (ASU), aims to help mitigate the urban heat island effect. It is testing the application of a treatment – made with asphalt, water, soap, mineral fillers, polymers and recycled materials – applied on top of the existing pavement.

Asphalt collects and retains heat during the day and releases it at night. Higher night-time temperatures lead to more energy consumption, greenhouse gas emissions and air pollution. The cool pavement coating reflects sunlight to reduce heat retention.

The first-year results showed that cool pavement surface temperatures are considerably lower than traditional roadway pavement. On average, treated pavements were 10.5 to 12 degrees Fahrenheit lower than traditional asphalt at noon and during the afternoon hours, and 2.4 degrees Fahrenheit lower at sunrise.  Night-time air temperature at six feet of height was on average 0.5 degrees Fahrenheit lower over cool pavement than on the non-treated surfaces.

“This is exactly what we were hoping for,” said Phoenix Mayor, Kate Gallego.

“The results are promising. While there’s more work to be done, it’s exciting to see a technology that has the potential to meet the demands of a growing desert city in a world where temperatures are constantly climbing.”

Results

In 2020, the city selected portions of eight neighbourhoods, one in each City Council district and part of the parking lot in Esteban Park, to receive cool pavement treatment. The asphalt coating was applied to areas already in need of pavement preservation.

Data collection included thermal imaging through helicopter flyovers and temperature sensors embedded in the pavement surface.

ASU researchers also developed a specially designed mobile weather station that evaluates the human experience of heat by measuring 3D mean radiant temperature, air temperature, relative humidity, and wind speed and direction. Additionally, a vehicle equipped with air and surface temperature sensors travelled over the treated areas to gather data during four times of the day.

Next steps

The study found that human experience of heat exposure at noon and the afternoon hours was 5.5 degrees Fahrenheit higher due to surface reflectivity, but similar to walking on a typical concrete sidewalk.

Further, surface solar reflectivity declined over ten months from a range of 33 to 38 percent to a range of 19 to 30 percent across all eight neighbourhoods. Untreated asphalt has a reflectivity of 12 percent.

“This project is an excellent example of innovation and collaboration,” Street Transportation Department Director Kini Knudson said. “City staff innovated a spray-application method that greatly reduced time and labour costs, and then collaborated with ASU to use applied science to study its effectiveness. While we continue to study the initial cool pavement installations, we will soon begin a second phase of testing using a darker material that has been engineered to have an even higher surface reflectivity.”

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