PHOENIX (3TV/CBS 5) -- There are a few things to understand about reflective or "white" pavement on roads. First, no one is saying, "paint all the streets white, and Phoenix will be cooler." But it could be an effective part of an overall strategy to combat the heat in urban areas. There's a lot that's unknown about the overall impact of lighter-colored streets because it's not being done on a huge scale anywhere, and that's necessary for it to work.
Here's what we can say about reflective streets. The surface of the street will be cooler because the solar energy will be reflected away. A study in Los Angeles showed the surface of the "reflective" streets was 10-15 degrees cooler during the summer months. The cooler surface has some potential benefits, including potentially allowing the road to last longer without repair. But then there's the problem of keeping the street ultra-white.
The sun's light is reflected and must go somewhere. Some recent studies have shown that someone walking along the sidewalk of a reflective street will probably be hotter because they're getting direct sunlight and reflective sunlight. Think how fast you get sunburned in a pool or on a ski slope. Some of the other reflected sunlight could make it into homes, cars, and buildings.
The good news is that some of that sunlight, because it's not being absorbed by any materials and turned into longwave radiation, can make it back out of the atmosphere. However, it appears the number would be small. All of the clouds around the planet reflect about 25% of incoming solar radiation back out into the atmosphere. The earth's surface itself reflects about 7%, but that's the whole planet, not just a network of streets.
So yes, reflective streets may help as part of a multi-faceted approach to easing urban heat. Planting trees is more effective in mitigating heat. But you can't plant a tree in the middle of Central Avenue and Camelback Road.