Analysis of Visual Adaptability in the Threshold Zone of Tunnels

Compare to the ordinary road sections, the traffic environment of tunnels is particularly different, especially when considering the visual environment. Due to the black hole effect, the threshold zone is regarded as accident-prone location. But in the engineering practice in China, the greatest change of the illumination in tunnels happened not at the tunnel portal but at somewhere inside the tunnel. This paper aims at discovering the distance range with the highest driving risk in threshold zone of tunnel. A field experiment has been conducted in a typical tunnel in Hunan Province, China. Based on the experimental data analysis, the relationship is built between the target detection distance, the distance from the tunnel portal, and the ratio of illumination, respectively. The results showed that at operating speed of 60 km/h, the distance range with the highest driving risk is 70 m to 109 m from the tunnel portal. The distance range is 50 m to 87 m from the tunnel portal for 80 km/h and 42 m to 79 m from the tunnel portal for 100 km/h. Based on target detection distance, the illumination design for tunnel lighting can be optimized.