Traffic signals include traffic lights and pedestrian walk signs.
Current standards for traffic signals were adopted by Congress in in EPAct 2005 and went into effect in 2006. The standards set 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. Yellow balls can use other lighting technologies since they have very low operating hours.
California first adopted state standards for traffic signals in 2002, effective in 2003. Eight additional states (AZ, CT, MD, NJ, NY, OR, RI, WA) adopted the CA standards, though all standards but those in Maryland were preempted by the national standards before taking effect.
Because the first LEDs produced monochromatic (one color) light, traffic signals were one of the first products to use them.