LEDs have been widely used in lighting applications, including traffic signs and traffic signals. Yet the design of LED traffic signs has no universal specifications. This study evaluates four custom-made LED highway traffic signs with different designs, including external LED lighting, LED backlight, regional LED backlight and LED lightbox. Participant observation studies were performed to investigate which design would have the best visual quality.
The ergonomic experiments use the four LED traffic signs as the experimental factor. The signage dimension is one-sixth of the real traffic signs, and the observation distance is 25 m. The luminance range of internal lighting signs is set at 200-350 nit, which complies with the “Technical Regulations of LED Road Traffic Signs” published by the Bureau of Energy, Ministry of Economic Affairs. The illuminance on the non-self-luminous sign with external LED lighting is set to 400 lux, within the range regulated by the “Road Traffic Construction Regulations” enacted by the Ministry of Transportation and Communications. We collected subjective scores from the participants, including the legibility, visual comfort, preferences and ranking.
The subjective scores were analyzed by the statistical software. The results showed that self-luminous traffic signs provide superior legibility, visual comfort, preferences and ranking than the non-self-luminous one. We also found the ambient illumination a key factor affecting the visual quality. Among the traffic signs self-luminous with LEDs, regional LED backlight is most susceptible to the ambient illumination because the green background is not lit. The green background relies on the ambient illumination to project on the retroreflective sheet to provide brightness. A small variation in the ambient illumination would result in tremendous change to the visual performance of regional LED backlight. Both LED lightbox and LED backlight light up the whole signage area and provide similar visual sensations to the participants, with the subjective scores of LED lightbox significantly better than that of LED backlight. Based on the four LED traffic signs designed and evaluated in this study, we suggest to use LED lightbox for future installations of LED traffic signs.
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