Cooling Pavements In Phoenix Could Come With An Unexpected Drawback

Phoenix, Arizona is no stranger to scorching, relentless heat, and the sprawling miles of dark asphalt crisscrossing the city only makes things worse, especially when it comes to the heat island effect. Traditional black pavement acts like a giant sponge for the sun's heat, soaking up radiation all day and releasing it after the sun goes down. To combat this, the city's planners started rolling out a seemingly ingenious solution: A water-based sealant (called CoolSeal) that coats existing dark-colored streets into a lighter, heat-reflecting grey. It's promising but has a major drawback. It's hotter for pedestrians to walk on.

At first glance, Phoenix's Cool Pavement program looks like a total game-changer for cities desperate to lower the thermostat, and goes a step beyond non-electric cooling systems for homeowners. The initial numbers sound like a big win for the cooling technology. By applying this special coating to existing roads, researchers learned it could drop the surface temperature of the asphalt by 10 to 12 degrees Fahrenheit during the hottest hours of the day. A cooler road surface can reduce maintenance needs over the years, saving local governments substantial money. 

Citizens and local officials alike were thrilled by the prospect of streets not feeling like a hot frying pan. Phoenix has already coated 200 miles of pavement with CoolSeal, and immediate surface-level results were exactly what engineers hoped to see. But the true test of any climate adaptation tool is how it affects the people living with it, and that is where this promising tech takes an unexpected turn.

To fully understand the catch this cooling solution comes with, you have to look at how the technology deals with the sun. Dark pavement absorbs solar energy, trapping heat inside the material. Grey-colored CoolSeal flips the script by reflecting incoming sunlight back up into the atmosphere. The pavement sees reduced temperatures, but all of that rejected thermal energy has to go somewhere. Unfortunately, that destination happens to be where pedestrians are often walking.

Because the heat is being reflected upward, instead of absorbed inward, anyone standing on or near treated roads gets caught in the crossfire. A study evaluating reflective pavements found that this upward reflection increases how humans experience that thermal stress, due to the mean radiant temperature, or how hot an environment actually feels. At midday, when the sun is beating down hardest, the environment just above the cool pavement was found to be hotter by about 7 degrees F when compared to standing on untreated asphalt. While the treatment lowers the pavement temperature, it doesn't make a midday stroll more comfortable for pedestrians.

So, does that mean the entire pavement cooling project in Phoenix is a bust? Not necessarily. Urban planners just have to be strategic about where they use this heat-reflecting technology. The coating does a good job of stopping heat from soaking into the ground, which is crucial for lowering ambient temperatures at night. The key is keeping this approach away from heavy foot traffic. Because that heat and radiation bounces off the treated asphalt, it'd be ideal for roads with low pedestrian traffic and unshaded parking lots. 

In these areas, the benefits of cooler surfaces and reduced nighttime heat should outweigh the midday spike in radiant heat. This way, cities can protect their roads and lower temperatures without affecting the residents. As cities worldwide search for ways to survive rising temps, this cool pavement experiment in Phoenix proves that there is no one-size-fits-all magic bullet. The best way to fight urban heat issues is with a variety of technologies specific to local conditions, like planting trees and even bizarre ideas for cooling your home, like a yogurt window coating.