A high power LED lamp and method for retrofitting conventional traffic signal lamps. The LED lamp includes a housing, a power supply disposed in the housing, a plurality of LEDs mounted to a substantially planar mounting surface in the housing and electrically connected to the power supply for producing diverging light, and a threaded electrical connector extending from the housing. The method includes replacing a convention incandescent light bulb with the LED lamp, and installing a Fresnel lens inside the traffic signal lamp that collimates and just fills and illuminates the outer lens of the traffic signal lamp.