"In the United States, dwellings tended to be built with white roofs through the 1960s. Then, as air conditioning became widespread, cheap, and taken for granted, priorities shifted. It became popular to use darker roofing shingles, which more resembled wooden shingles and better concealed dirt and mold. The colored granules on typical "white" shingles made today are coated with only one-sixth as much white pigment as in the 1960s. Under the summer sun, modern shingles become 20°F hotter than the old-style ones.
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Increases in temperature do not have to follow from an influx of population. The Los Angeles basin in 1880 was still relatively barren, and yearly highs ran about 102°F. Then settlers introduced irrigation, the fruit trees cooled the air, and, within 50 years, summer temperatures dropped 5°F. But as Los Angeles began to urbanize in the 1940s, cool orchards gave way to hot roofs and asphalt pavements. Over the next 50 years, summer highs climbed back to their 1880 values-and are still rising at 1°F per decade, with no end in sight."
http://eetd.lbl.gov/HeatIsland/PUBS/PAINTING/
Los Angeles yearly high temperatures:

Further quote from article:
"This indirect annual savings would total an additional 12 percent-0.7 billion kilowatt-hours, or $70 million. As shown in the table below, implementing these cool community measures would lower the need for peak electrical generating capacity by about 1,500 megawatts-equivalent to two or three large power plants.
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The cooler temperature would lower smog, too.
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Thus a 12 percent reduction in ozone exceedance could save $360 million." [in just health-related costs]