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LED TRAFFIC SIGNAL MONITORING, MAINTENANCE, AND REPLACEMENT ISSUES SUMMARY Light-emitting diode (LED) traffic signal modules were first widely used in the 1990s because of their significant energy savings and their much longer service life relative to incandescent signals. The Energy Policy Act of 2005 mandated that traffic signal heads manufactured after January 2007 achieve energy consumption levels consistent with LED technology, effectively making obsolete any further manufacturing of incandescent traffic signals. Although LEDs are being implemented on a widespread basis, there are concerns regard- ing monitoring, maintenance, and replacement of LED modules. These concerns are centered on several factors including long-term degradation of light output, their mode of failure, and issues, such as cost, associated with their replacement. This synthesis includes the results of a 2006 Institute of Transportation Engineers (ITE) Task Force survey effort of users and vendors/manufacturers of LED modules. The specific objectives of the synthesis are to familiarize readers with the history of LEDs as replacements for incandescent lamps, to facilitate an understanding of LED issues, to document lessons learned, and to present successful practices to minimize future problems. ITE survey information presented includes 75 responses from public agency traffic engineers, consisting of ITE public agency members and AASHTO state traffic engineers, and 6 LED vendors/manufacturers [members of the National Electrical Manufacturers Asso- ciation (NEMA)]. These survey responses were supplemented by follow-up discussions with various state agency officials who confirmed the initial survey results. A case study details more specific LED information gleaned from the experience of the Road Commission of Oakland County (RCOC), Michigan, with LEDs dating back to 2000. The synthesis identifies technical issues associated with the operation and monitoring, maintenance, and replacement of LED traffic signal modules. It also suggests some best prac- tices to deal with the complexities of this new and evolving technology. It concludes with suggestions for additional research to resolve outstanding technical issues.