Safety Evaluation of Illuminance Transition at Highway Tunnel Portals on Basis of Visual Load

Highway tunnel entrances and exits are accident-prone locations, especially in the daytime, primarily because of the sharp transitions in lighting in these areas. Therefore, a rational assessment of lighting transitions at tunnel portals is the key to ensuring traffic safety. This study used the EMR-8B eye-tracker system to monitor the pupil changes of eight selected drivers as they drove through 26 typical highway tunnels. The test results showed a power function relationship between drivers’ pupil areas and pupil illuminance at highway tunnel portals. A quantitative relationship between the pupil area and its critical velocity was also established, and the ratio (k) of the pupil area's changing rate in relation to its critical velocity was used to assess the visual load and to further evaluate the lighting transitions. The results demonstrated that (a) the relationship between pupil illuminance at the tunnel portal and the driver's pupil area conformed to Stevens’ law from experimental psychology, (b) the relationship between the driver's pupil area and its critical velocity followed the quadratic function, (c) the visual load at a tunnel entrance was heavier than that at the tunnel exit, and (d) the severe transitions in pupil illuminance within 10 m of some existing highway tunnel entrances caused a great visual load, and thus are urgently in need of improvement.