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'Cool pavement' technology shows promise to help reduce heat in the streets

Asphalt road empty sidewalk
Posted at 1:07 PM, Oct 08, 2021

July 2021 was the earth’s hottest month on record, according to data from the National Oceanic and Atmospheric Administration (NOAA). The land and ocean surface temperatures were a combined 1.67 degrees above the average.

One city in the southwest region of the U.S. is looking at solutions to help keep the ground cooler.

“It looks basically like cured concrete,” said Ryan Stevens, a civil engineer with the City of Phoenix.

Ryan Stevens and other engineers are trying to lower the temperature of the asphalt.

“Really, it gives that illusion of putting on the product like sunscreen onto our roads, and that's really what it's doing is insulating our roads from the daytime solar energy," he said.

In nine areas across Phoenix, the city is piloting its "Cool Pavement Program." It’s a lighter coating on top of the roads already in place.

“It’s not paint. We hear people say, 'Well, you just painted your streets,' we didn't paint our streets. We actually used an asphalt-based product,” said Kini Knudson, director of Street Transportation for the City of Phoenix.

The program began last year and currently coats 36 miles of asphalt across the city.

“If it’s something that makes sense from an environmental climate perspective but also makes our pavement last longer, for me, it's a no-brainer,” he said.

“The idea is to keep the energy out of the pavement because at night the pavement is the last thing to cool down. About 30 to 40 percent of the urban area is paved surfaces,” Stevens said.

Paved surfaces contribute to similar problems in cities across the U.S. Larger populations and more buildings created a higher urban heat island index -- it’s a concept called the urban heat island effect.

“The urban heat island is a situation that has been studied for about 200 years. It happens because you have buildings that can store heat differently than the natural surface. So a building will hold onto heat and hold it on through the night, and what that does is it keeps the nighttime temperatures warmer so we have this situation where we’re not cooling off at night,” Erinanne Saffell, Arizona’s state climatologist, said.

“That energy, the sunlight comes down and is stored in the surfaces,” she explained. “The more reflective a surface, the less energy it’s going to store and so it's not going to be as hot.”

Saffell compares it to how a person might feel wearing a dark t-shirt in the hot sun.

“If you're wearing a light-colored shirt that reflects the heat, that's not going to get you as hot,” she said.

“We know that heat is a challenge for our community, we want our community to be comfortable,” Kate Gallego, the Mayor of Phoenix, said. “We’ve noticed it’s not getting as cool as night, we’re retaining a lot of heat.”

Results from the first year of the program show the cool pavement had an average surface temperature of 10.5 to 12 degrees Fahrenheit lower than regular asphalt at noon and during the afternoon hours. At all times of the day, the cool asphalt was cooler than traditional asphalt.

“That’s enough temperature distance or change that you can feel it if you're walking in a community,” Gallego said.

However, Stevens said it’s still being looked at how this impacts the air around the asphalt.

“The degree of which that air mixing, does it feel hotter or not, is something we still need to look at,” he said.

The city plans on trying different colors and shades as well.

Keeping buildings and asphalt lighter is something Steve Hamburg says has been used in design decisions for quite a while.

“If you go to the Mediterranean, you would never see dark roofs. You're always going to see light roofs, because they want to reflect the heat off the roofs so it doesn't warm the structure…people have been known to do that for thousands of years,” Steven Hamburg, chief scientist at the Environmental Defense Fund, said.

The project in Phoenix will continue to see what works best to mitigate the urban heat island effect and keep our planet cooler.

“It’s going to take a few more years to be able to fully evaluate the impact and the benefits of it,” Knudson said.