Expressway visual distance detection using thermal imager: Comparing normal and infrared images

The real-time monitoring of visibility for expressway traffic safety cannot be widely achieved due to the high cost of the traditional method. This paper proposes a novel method to monitor expressway visibility using two types of cameras, one normal and one infrared. The former is affected by visibility conditions when the latter is not. The concept of visibility level (VL) is proposed correspondingly, based on a comparison model of normal and infrared images. The mapping relation of the ratio of two kinds of image edge pixels to the decrease of visual distance is determined, establishing the VL model. Camera correction coefficients and temperature correction coefficients are proposed to make the model more reasonable and accurate. The model is validated in experiments under foggy conditions. The results show that the average error of the VL model is 10%. The research may enable practical implementation on roadsides in the near future to determine visibility for the benefit of expressway safety.