A traffic signal backplate surrounds a light to help it stand out against the sky or background. An option in the MUTCD allows departments of transportation to add a yellow retroreflective borders. A 1998 study conducted by the Insurance Corp. of British Columbia and the Canadian National Committee on Uniform Traffic Control, investigated the effectiveness of applying retroreflective tape around the borders of traffic signal backplates. The investigation concluded that the enhancement was effective at reducing crashes.
Following the Canadian study, yellow backplates were installed at three intersections in Columbia, South Carolina. An intersection safety case study revealed that the three locations experienced a 28.6 percent reduction in total crashes during a 25 month period.

An added benefit is that the retroreflective borders add visibility to signal heads during power outages. This draws extra attention for motorists to treat the dark signals as a four-way stop.

Source: “Traffic signals on Clairemont Avenue get shiny new look.” Leader-Telegram (Eau Claire, WI), December 28, 2013.