Washington Post: One of the latest hot spots in Washington’s fight against global warming hangs 25 feet above the speed humps and manses of the little town of Somerset in Montgomery County — 174 streetlights bolted to wooden telephone poles, a prosaic government service tucked under the tree canopy. No one ever paid much attention to the humble street lamps, until Pepco announced last summer that it was replacing the mercury vapor bulbs in 36,000 fixtures in the Maryland suburbs with a more …