How many traffic planners does it take to screw in a lightbulb? The answer to that riddle is worth 40-thousand dollars a year to Iowa City. Engineer Jeff Davidson says the city’s replacing all its green lights with new, energy-efficient bulbs.A few years ago they changed out all the red bulbs with the energy efficient bulbs. There wasn’t money to change all the lights back then, but now Davidson says prices for the new lights have come down and a traffic-light replacement program’s available through a subsidiary of Mid-American Power Company.Davidson adds while an old-fashioned light bulb is changed every year, the new LED lights are guaranteed for seven years and expected to last even longer. You may be looking at new traffic lights already — look for what seems to be a collection of many small lights in the red or green circle.