China and US researchers develop new material for color-changing road signs

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Researchers from two universities in China and the USA have developed a new thin retroreflective film that reflects light in intriguing ways, which could be used to make road signs that shine brightly and change color at night.

The research was led by Limin Wu, at Fudan University in China, whose group developed the material. Experts on optics at the University at Buffalo (UB) in the USA made significant contributions to the work, providing insight into potential applications for the film, such as employing it in nighttime road signs. The researchers say the new technology could help call attention to important traffic information when it is dark, with potential benefits for both drivers and pedestrians. The film consists of polymer microspheres laid down on the sticky side of a transparent tape. The material’s physical structure leads to an interesting phenomenon: when white light shines on the film at night, some observers will see a single, stable color reflected back, while others will see changing colors. It all depends on the angle of observation and whether the light source is moving.

In one set of experiments, researchers created a speed limit sign with letters and numbers made from the new retroreflective structural color film (RSCF). The scientists placed a white light nearby to illuminate the sign, and when a fast-moving car drove past, the color of the characters on the sign appeared to flicker from the perspective of the driver as their viewing angle changed. In other tests, the team applied the new material to a series of markers lining the side of a road, denoting the boundary of the driving lane. As a car approached, the markers lit up in bright colors, reflecting light from the vehicle’s headlights. From the driver’s perspective, the markers’ color remained stable. But to a pedestrian standing at the side of the road, the color of the markers appeared to flicker as the car and its headlights sped past.

“You can use this material to make smart traffic signs,” said Qiaoqiang Gan, an associate professor of electrical engineering in the UB’s School of Engineering and Applied Sciences and a co-first author of the new study. “If a person is listening to loud music or isn’t paying attention while they’re walking or driving, a color-changing sign can help to better alert them to the traffic situation.”

Co-author Haomin Song, UB assistant professor of research in electrical engineering, added, “If the car goes faster, the pedestrian will see the color change more quickly, so the sign tells you a lot about what is going on.”

The roadway experiments were performed in China, but Gan, Song and UB graduate, Dengxin Ji, helped design the tests. In addition to his position at UB, Gan was a summer visiting professor at the University of Shanghai for Science and Technology during the period of the collaborative work. The study was funded by the National Key Research and Development Program of China and China’s National Natural Science Foundation.

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Adam joined the company in 1994, and has been News Editor of TTT since 2009. In his other role as Circulation Manager, he helped create the original Traffic Technology International distribution list 23 years ago, and has been working on it ever since. Outside of work, he is a keen fisherman, runs a drumming band, and plays an ancient version of cricket.

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