Traditionally, traffic signals use long-life incandescent bulbs. However, these lights have high operating hours, making energy use high.
In recent years, LED (light emitting diode) traffic signals have been introduced that reduce energy use by about 90% relative to incandescent lights. The LED signs were promoted by the Energy Star program and many utility and state programs. California and other states adopted standards, setting watt caps of 8-13 watts for each ball (red or green) or face (e.g., walk signal) when tested at 25 degrees C. This essentially requires use of LED lights. Congress made this a national standard in 2005, with the standard taking effect in January 2006. Yellow balls can still be incandescent since they have very low operating hours.