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AZ Shares Cool Pavement Research Results

Wednesday, September 22, 2021

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During a virtual presentation and panel discussion last week, the City of Phoenix Street Transportation Department and Office of Sustainability shared the results for the first year of its Cool Pavement Pilot Program.

The program and analysis of the cool pavement process is being conducted in partnership with Arizona State University.

Cool Pavement Pilot Program

The idea for the pilot study was launched following a ASU-led study on solar reflective coatings back in July 2019. For the study, ASU researchers took two mobile biometeorological instrument platforms (MaRTy 1 and MaRTy 2) to Los Angeles in order to read how the city’s streets affected radiant heat and ultimately, pedestrians’ comfort.

At the time, the Los Angeles Bureau of Street Surfaces was pioneering the use of solar reflective coatings in a quest to cool city streets.

In the study’s findings, researchers found that the temperature of the coated asphalt road was up to 6 degrees C (almost 10.8 degrees F) cooler than the regular road in the afternoon. However, the radiant heat over coated asphalt was 4 degrees higher than non-coated areas. Additionally, the study also found that the coating didn’t have a big impact on air temperature—only half a degree in the afternoon and 0.1 degree at night.

“We need more of these experiments,” Ariane Middel, assistant professor in both the School of Geographical Science and Urban Planning and the School of Arts, Media and Engineering at ASU said at the time. “There have been a lot of large-scale modeling studies on this. So, we don’t know in real life if we get the same effects. The urban environment is so complex, and models have to always simplify. So, we don’t know what really happens on the ground unless we measure, and there haven’t been these types of measurements in the past.”

The research study, “Solar reflective pavements — A policy panacea to heat mitigation?” was published in April of this year in the journal Environmental Research Letters.

In developing the Cool Pavement Pilot Program from the study, the City of Phoenix Street Transportation Department selected portions of eight neighborhoods and one city park to receive cool pavement treatment as part of the project. The portions covered 36 street miles at a cost of $3.3 million, which was provided by the city’s street maintenance budget.

City of Phoenix

During a virtual presentation and panel discussion last week, the City of Phoenix Street Transportation Department and Office of Sustainability shared the results for the first year of its Cool Pavement Pilot Program.

The nine areas chosen were already due for resurfacing and money would have gone toward rehabilitating those areas regardless of the material used, reported David Sailor, Director of the Arizona State Urban Climate Research Center.

However, instead of using the reflective solar coatings like in Los Angeles, the city used a cool pavement technology, CoolSeal, developed by California-based GuardTop LLC.

According to the Department, the application is made up of asphalt, water, an emulsifying agent (soap), mineral fillers, polymers and recycled materials. It contains no harmful chemicals, is compatible with traditional asphalt and can be spray- or squeegee-applied.

The city added that it chose to use this form of cooling technology as opposed to the reflective coatings because, “Cool pavement reflects back the sunlight that hits it. Because the surface reflects rather than retains heat, cool pavement has the potential to offset rising nighttime temperatures in Phoenix. The use of cool pavement technology may help reduce the heat island effect and reduce temperatures in the city. It is also useful to cool neighborhood areas that don’t have much shade from the sun.”

Since announcing the program’s endeavors, the city was expected to complete the application of cool pavement coatings on all its locations by the end of October 2020.

While a team from ASU plans to study the cool pavement’s effects over the next several years, early results were slated to be posted on the Street Transportation Department website by the end of 2021.

In addition to the Street Transportation Department and ASU, the City of Phoenix Office of Sustainability have also partnered on the project to see if the technology can help with the city’s continuing efforts to be environmentally sustainable while improving the quality of life for all who enjoy Phoenix.

First Year Results

According to scientists from ASU’s Global Institute of Sustainability and Innovation, Healthy Urban Environments, and the Urban Climate Research Center, the reflective pavement surface temperatures were considerably lower than traditional roadway pavement.

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

Additional findings from year one of the study include:

  • Cool pavement revealed lower surface temperatures at all times of the day versus traditional asphalt;
  • Cool pavement had an average surface temperature 10.5 to 12 degrees Fahrenheit lower than traditional asphalt at noon and during the afternoon hours. Surface temperatures at sunrise averaged 2.4 F lower;
  • Sub-surface temperatures averaged 4.8 F lower in areas treated with cool pavement;
  • Nighttime air temperature at six feet of height was on average 0.5 F lower over cool pavement than on the non-treated surfaces;
  • The human experience of heat exposure at noon and the afternoon hours was 5.5 F higher due to surface reflectivity, but similar to walking on a typical concrete sidewalk; and
  • Surface solar reflectivity declined over 10 months from a range of 33% to 38% to a range of 19% to 30% across all eight neighborhoods. Untreated asphalt has a reflectivity of only 12%.

To collect date for the study, ASU researchers were reported to have used thermal imaging through helicopter flyovers, temperature sensors embedded in the pavement surface and other advanced instruments. The team was also reported to have developed MaRTy (derived from ‘Mean Radiant Temperature’), 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.

For additional measures, researchers also deployed a vehicle equipped with air and surface temperature sensors traveled over the treated areas to gather data during four times of the day.

“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 labor 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.”

In concluding year one’s study results, the City of Phoenix reported that it is one of several cities currently experiencing the urban heat island effect, particularly overnight, due to the retention of heat within the built environment.

Higher nighttime temperatures lead to more energy consumption, more greenhouse gas emissions, air pollution and other harmful effects.

To read the full Executive Summary of ASU’s scientific study, click here.

Editor's Note: The article was edited at 8:13 a.m. on Sept. 22, 2021, to reflect the correct difference in degree temperatures.


Tagged categories: Asphalt; Coating Materials; Coatings Technology; Colleges and Universities; Cool Coatings; Department of Transportation (DOT); NA; North America; Research; Research and development; Roads/Highways; Solar reflectance

Comment from William Cornelius, (9/22/2021, 8:10 AM)

"The temperature of the coated asphalt road was up to 6 degrees C (almost 43 degrees F) cooler than the regular road in the afternoon" came from the original California article and is corrected in the additional findings part of the story. In fact, 6°C is about 43°F on a thermometer. However, since 1°C = 1.8°F the six degree difference is 10.8°F.

Comment from Destiny Johnson, (9/22/2021, 8:16 AM)

Thank you for this correction, William. The article has since been edited to reflect the correct difference in temperature. We appreciate your readership and hope you have a great day!

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