低能见度是高速公路安全事故的重要诱因之一，而传统能见度检测仪架设方式复杂、价格 昂贵，不利于大范围推广，为了实现全路段低能见度的实时监测，提出了一种基于已有监控图像 的低能见度可视面积比检测法。普通图像可视景物的面积随着能见度的降低而减小，因此根据空 间几何投影关系，由图像可视景物的面积比来推算能见度的大小，同时引入红外图像作为处理普 通图像的参照，简化图像处理算法，降低对普通图像的质量要求，增加检测法的实用性。模拟试 验结果显示，只考虑对高速公路驾驶安全影响最大的200m范围内的低能见度，引入红外图像的 可视面积比检测法能有效提高检测精度，且平均误差小于10%，故可以经济全面地用于高速公路 低能见度的实时监测，以提高行车安全性。

Low visibility is one of the inducing factors for expressway accidents. Traditional visibility detection devices are usually erected in a complex manner, and are expensive and difficult to be popularized. In order to real-time monitoring the low visibility of whole expressway, a new detection method based on the low visibility visual area proportion of existing monitor image was proposed. Usually, the visual scene area of image reduces with the decrease of its visibility. Therefore, according to the geometric projection relation, visibility could be estimated according to the visual area proportion of image. Mean⁃ while infrared image was used as the reference of common image, which could simplify the processing algorithm, reduce the quality requirement of the common image and improve the practicality of the detection method. Simulation experiment showed that the visual area proportion detection method of infrared image can improve the detection precision, for we only consider the low visibility in the range of 200m, which can greatly affect the driving safety of expressway, and the average error is less than 10%. Therefore, it can be used for high speed real-time monitoring of low visibility of expressway to improve traffic safety.