"Adam Fischer, transportation director for the city of Greensboro, said that if the city converted all of its approximately 26,000 incandescent street lights to LED (light-emitting diode) lights, the energy savings would be about 55 percent, or 11.9 million kilowatt hours annually. The city's conversion would reduce coal ash production by 400 to 600 tons per year, he said."
As the linked article points out there is some upfront costs but the entire system need not be switched out all at once and long term life spans and energy savings will make up the difference.