Why aren't the high-speed gantry-mounted spotlights canceled?
4 Answers
Gantry structures don't significantly impact safety and serve irreplaceable functions, hence they haven't been removed. Gantries provide supplementary lighting: The spotlights mounted on gantries, also called fill lights, enable the license plate capture systems installed on these structures to take clearer photos with more substantial evidence. As the name suggests, highways involve higher vehicle speeds, where traffic violations become more dangerous when occurring at such velocities. Meeting illumination requirements: Fill lights should ensure uniform illumination within their coverage area, without dark spots or noticeable flickering. At 20 meters from the lighting device, the peak illuminance on the reference axis should be below 300lx, with average illuminance under 50lx, and visible light color temperature between 3000-5500K. This brightness level is roughly equivalent to that of an ordinary streetlight.
From actual driving experience, those glaring spotlights on highway gantries may be annoying, but they're definitely not installed without purpose. Once during a long night drive, I encountered thick fog where the white reflective panels ahead became completely invisible. It was only through the purple-blue lights projected from the gantry that I could identify the lane markings. These lights primarily serve to capture speeding and lane violation offenses, offering significantly higher precision than regular surveillance cameras - especially crucial for nighttime license plate recognition through reflective identification. Some regions are experimentally replacing the intense lights with infrared monitoring, but fog penetration becomes substantially worse. If these lights were to be removed, the accuracy rate of e-tickets would likely drop by about 30%, probably requiring additional gantry installations to compensate.
I used to wonder why these floodlights weren't upgraded, until I asked a friend who works in highway maintenance. These white lights are actually specially designed with a peak wavelength of 650 nanometers to precisely match the reflective coating on license plates. In low-light nighttime conditions, ordinary car headlights make license plates appear blurry in camera footage, like water-damaged photos. Newly constructed smart highways are now using low-light laser cameras, but retrofitting thousands of kilometers of existing roads would be prohibitively expensive. It's said that Jiangsu-Zhejiang-Shanghai region tested LED cold light sources as replacements, but they lacked sufficient penetration during rainy or foggy weather - failing to illuminate when needed most.
From a technical perspective, gantry-mounted lighting is irreplaceable. Regular LED lights experience significant attenuation beyond 20 meters, whereas these customized spotlights maintain stable illumination even at 60 meters. The key lies in dynamic adaptation to license plate reflection: when vehicles pass at high speed, the instantaneous fill lighting must precisely illuminate the plate without causing overexposure. While LiDAR could theoretically replace this function, signal conflicts arise during multi-lane concurrent capture. Some sections of the Shanghai-Nanjing Expressway have now been upgraded with composite fill lights, reducing energy consumption by 40% while minimizing glare—representing a balanced solution.
