Using Different Colored Streets To Keep Our Cities Cool

All the black asphalt roads capture tons of heat, making cities sweltering in the summer, which in turns makes them more expensive to cool. But who said streets had to be black? Researchers at Lawrence Berkeley Lab are trying to find a different way.

If you were going to design a cheap way to trap a lot of heat in cities, a standard asphalt pavement would be a pretty good choice. The mixture of black rocks and gooey black stuff holding it together is an excellent invention if you want to absorb as much sunlight as possible, and re-radiate that energy as heat.

Except, most cities don’t want to absorb as much sunlight as possible. They want to cool down, not get steadily hotter.

To show that there are alternatives to hot asphalt, Lawrence Berkeley Lab, in California, currently has a showcase of "cool pavements" that are designed to reflect between 30% to 50% of the energy, compared to about 5% for conventional surfaces. On some days, according to Benjamin Mandel, a researcher with the Heat Island Group, the new coatings are 40 degrees Fahrenheit cooler than traditional counterparts.

The coatings, which are made by several private companies, come in various shades and colors, and are currently displayed in Berkeley’s (very dark colored) parking lot. Mandel says they’ve tested eight so far. "Our showcase is intended primarily for demonstration purposes, as we show the public and local governments some cool pavement options that are currently on the market," he says.

Mandel says it’s not possible to say which of the eight is best, as they have varied uses. "Some likely perform better under vehicle traffic on city roads. Others may maintain solar reflectance better over time."

But, with more than a third of cities taken up with pavement, using cooler coatings could have a big impact on reducing air temperatures and improving air quality. Studies have shown that lighter surfaces, combined with more vegetation, could impede the formation of smog, and reduce energy costs—for example, from air conditioning.

Mandel says the coatings are likely to be a little more expensive up-front. But this could be offset by savings from longer-lasting pavements, and better preservation of the "system" underneath. If it’s a little easier to walk around in the summer, it sounds good to us.

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  • Dwayne D'Ardenne

    No debate that there are wonderful benefits from pavements reflecting solar energy, but need more data on longevity/life cycle costs of cool pavement coatings. Experience tells me that coatings will simply mean additional long-term maintenance/life cycle costs. Most localities continue to struggle with how to balance reduced street maintenance budgets with continually rising asphalt costs - result has been and continues to be nationwide roadway ratings worse than they have been in decades. The upshot is that public works folks will likely not have funds for maintenance of cool pavement coatings. Ideally, research will identify/create asphalt design mixes that increase solar reflectivity for the life of the pavement without significantly increasing upfront costs...

  • Bob

    So where does the heat go with these lighter pavements? This study is looking only at surface temperatures, but what about things next to the parking lot? It seems that if heat is reflecting off these surfaces it's going to warm up things near by (cars, buildings, people, etc.). 

  • Christopher Reader

    When light is absorbed, the energy in the light is converted to heat. When light is reflected, the energy is not absorbed. Perhaps some of the light would be reflected onto objects which might make them very fractionally warmer, but most of the light would be reflected into "space."