Roads in Phoenix could be up to 12 degrees cooler if the city paves all of its asphalt roads with a lighter coating, according to a pilot program launched by the Office of Heat Response and Mitigation.
Nearly all of Phoenix’s roads are asphalt, according to Ryan Stevens, an engineer with the city. Left as they are now, the roads absorb a lot of heat which adds to Phoenix’s rising temperatures.
A 2021 ASU study finds normal asphalt roads in the city reflect just 12% of all sunlight.
The Office of Heat Response and Mitigation is looking to change that with its Cool Pavement Program. With help from Arizona State University, the city is testing new roads that could reflect as much as 40% of heat back into the atmosphere.
The solution scientists have reached is adding a layer of coating on top of the asphalt to prevent heat absorption. City leaders are looking to preserve the asphalt base because of the material’s recyclability and durability.
Roads are being designed to avoid what experts call the urban heat island effect.
The urban heat island effect makes cities hotter than surrounding rural areas. It occurs when asphalt roads absorb heat during the day and retain it at night.
Stevens is an engineering manager for the Street Transportation Department who says the heat is a problem.
“As the sun has gone down, the temperature cools, the asphalt is remitting heat, and it can make the environment hotter than the natural desert (at night),” Stevens said.
According to Kamil Kaloush, a professor at the School of Sustainable Engineering and the Built Environment, asphalt is not the only problem for heat absorption.
“It is not a matter of asphalt or concrete, it’s a matter of how we design it and how we optimize, how we want the material to behave,” he said.
The first thing the group did to test the coating was alter the albedo, or reflectivity of the road. The higher the albedo, the less heat a road absorbs. Scientists created a pavement coating with a 40% albedo using a product called CoolSeal. Phoenix paved nine different neighborhoods in 2020 to evaluate this mixture, which is also being tested by the city of Los Angeles.
On paper, the numbers revealed that the road would make an impact, but scientists quickly discovered they had added too much of a reflective property, causing drivers to have a difficult time actually seeing the roads.
Dr. Jose Medina, a research professor at Arizona State University, said, “Forty percent is too light, (there is) too much glare”
Now scientists are in phase two of trying to make roads cooler. Dr. Medina said engineers and scientists are trying to figure out the best way to reduce pavement temperatures without making roads too reflective.
Between May and June, the city paved sections of eight new neighborhoods with a new coating that has reduced the albedo level. Before scientists make a decision on the best version of the pavement they will wait 18 months to ensure the road can survive both a winter and summer seasons of driving.
The city also has sites selected for four more neighborhoods in 2023, but have yet to begin the paving process. A full list of where sites are planned can be found on the Phoenix Cool Pavement Projects dashboard.
Scientists want to make the right decision when selecting the best mixture. CoolSeal costs around $5 dollars per square yard of supply, but it can cost up to $14 dollars a square yard when applying the coating due to labor costs, according to Ryan Stevens.
The coating has the potential to last up to eight years before it needs to be re-coated. The sealant mixture on the roads before this program needed to be replaced every three to five years, according to Stevens.
The Office of Heat Response and Mitigation emphasized that the Cool Pavement Program is not the only solution to Phoenix’s rising temperatures.
“Coating all the roads is not a solution,” Dr. Medina said. “It has to be like an integrated solution of many other factors. Not just the roads, but shading trees, reflective roofing, (and) many other solutions.”
Contact the reporter at pdalal6@asu.edu
