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Publication Number: FHWA-HRT-13-070
Date: August 2013


Safety Evaluation of Converting Traffic Signals From Incandescent to Light-Emitting Diodes

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Across the Nation, many agencies have been replacing conventional incandescent light bulbs in traffic signals with light-emitting diodes (LED) (see figure 1 and figure 2). LEDs are primarily installed to reduce energy consumption and decrease maintenance. In addition, LEDs are expected to last much longer compared with incandescent bulbs and tend to age gradually.(1) However, a recent study revealed several potential problems with LEDs, including their inability to melt snow and issues related to visual discomfort caused by glare at night.(2)

Two recent studies, one in Middleton, OH, and the other in Memphis, TN, evaluated the safety effects of converting from incandescent bulbs to LEDs.(3,2) The Ohio study concluded that the LED conversion resulted in a 71-percent increase in crashes, while the Tennessee study concluded that the LED conversion resulted in a 47-percent increase in crashes. Both studies used before–after empirical Bayes (EB) evaluation techniques, but methodological issues associated with these two studies, which are discussed by Srinivasan et al., necessitate a cautious approach to interpreting and applying the results.(4) One of the most significant issues that these studies have in common is a very limited sample size—both studies use data from only eight intersections where incandescent bulbs were replaced by LEDs and two comparison/reference sites.


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United States Department of Transportation - Federal Highway Administration