As an alternative to incandescent light sources, light-emitting diodes (LEDs) offer tremendous energy and cost savings. To date, LEDs have been applied for signage, brake lighting, exit signs and increasingly in traffic lights. Traffic signals that use LEDs as a light source, offer state and local jurisdictions significant energy savings (approximately 80 to 90%), additional cost savings (from ... [Show full abstract] reduced maintenance), and the potential for improved visibility and safety. Some regions in the U.S. and Europe are taking advantage of these savings, but high product costs and lack of information limit market penetration in other regions. Additionally, the lack of a final LED traffic signal specification from the key U.S. standard-setting body or similar organizations world-wide, has hindered jurisdictions' pursuit of LED traffic signal retrofits. This paper reviews LED traffic signal installation activity to date, presents key technical and market barriers that limit greater market penetration, and summarizes five market transformation efforts that strive to address these barriers. In particular, the authors highlight: the development of LED traffic signal specifications; a U.S. EPA/DOE ENERGY STAR ® labeling program for traffic signals (in development) 1 ; the Consortium for Energy Efficiency's (CEE's) LED Traffic Signal Initiative; New York State's Energy $mart Program, and the International Energy Agency (IEA) DSM Annexe III LED Traffic Signal Procurement. Although the approaches may vary, each of these efforts generally aims to increase comfort with, and awareness of, LED technology among local decision-makers. Together these activities offer significant potential to build lasting demand for LED traffic signals.