Philadelphia painted black pavement with a light-coloured coating to fight extreme heat. A year later, researchers found one clear difference

On a hot summer afternoon, black asphalt can become considerably hotter than the air above it, absorbing sunlight and holding onto that heat long after the sun begins to set. Philadelphia is testing whether changing something as ordinary as the colour and surface of pavement could make a difference. Since 2024, the city has been experimenting with "cool pavement" in Hunting Park, replacing the conventional appearance of dark asphalt with a light-coloured reflective coating. The idea sounds straightforward, but the first year of testing has produced a more complicated answer: the pavement itself stayed cooler, while the effect on surrounding air temperature remained uncertain.

Why Philadelphia Is Experimenting With Its Pavement

Extreme heat does not affect every part of a city equally. According to the City of Philadelphia, temperatures in some vulnerable communities can be nearly 30°F warmer than in other areas. One contributor is the "urban heat island effect". Built-up neighbourhoods filled with roads, roofs and other dark surfaces can absorb solar energy during the day and release stored heat later. Areas with fewer trees and plants can be particularly vulnerable.

In summer 2024, Philadelphia's Office of Sustainability and Streets Department launched a Cool Pavement pilot in partnership with CoolSeal by GuardTop. Instead of replacing the pavement, workers applied a light-coloured reflective sealant over an existing section in Hunting Park. The material is designed to absorb less heat and reflect more sunlight than conventional dark pavement.

Researchers Wanted to Know More Than How Hot the Road Became

The experiment isn't simply about measuring pavement temperature. Researchers from the University of Pennsylvania's Composto Research Group and Thermal Architecture Lab are examining several factors, including surface temperature, air temperature and mean radiant temperature, an important measure of the heat people experience from surrounding surfaces.

Philadelphia experiences both hot summers and freezing winters, meaning the coating has to withstand regular traffic and cleaning as well as freeze-thaw cycles and road salt used during icy weather.

What Happened After One Year?

The first phase produced an encouraging result but also raised another question. Compared with untreated dark asphalt, the CoolSeal-coated pavement took longer to heat up and remained cooler from morning into the afternoon, according to the City. But cooler pavement did not automatically translate into measurably cooler air.

Researchers found no significant difference between air temperatures above the treated pavement and those above conventional dark asphalt. As a result, the project's ability to reduce surrounding air temperatures remained inconclusive after the initial phase. That distinction is important. A surface can become substantially cooler without necessarily cooling an entire outdoor environment by the same amount.

Philadelphia Is Now Testing Two Coats

Rather than ending the experiment, the City expanded it. Phase Two began in August 2025, when the Streets Department applied two coats of CoolSeal to pavement in Hunting Park. Researchers are now studying whether the additional coating improves cooling performance while also monitoring how well the material survives everyday use and Philadelphia's changing seasons. The broader question is whether reflective pavement could eventually become another practical tool for communities facing extreme summer heat.

A Simple Idea Facing a Real-World Test

Cool pavement will not replace trees, shade or broader efforts to prepare cities for rising temperatures. Philadelphia's own preliminary findings show why these experiments need to be studied carefully: making asphalt cooler and making the air around people cooler are not necessarily the same thing. Still, the Hunting Park experiment demonstrates how cities are reconsidering even the most familiar pieces of infrastructure as they adapt to extreme heat. For now, Philadelphia has established one thing clearly changing the pavement's surface can keep that pavement cooler. Phase Two will help determine whether doubling down with another coat can turn that simple effect into a meaningful benefit for the people walking, playing and living around it.