Drivers’ visual comfort at highway tunnel portals: A quantitative analysis based on visual oscillation

The paper analyzes the phenomenon of driver’s transient blind period by defining the concept of visual oscillation and uses it to evaluate the visual comfort degree at highway tunnel entrances and exits in China. On the basis of twenty-six typical highway tunnels and ten drivers tested, a large number of experiments on drivers’ pupil variation have been carried out within the range of 50 m from inside of tunnel portals. By defining the transient blind period as visual oscillation, the equivalent duration of visual oscillation (EDVO) is proposed for quantitative evaluation of the transient blind period and further reasonable evaluation of drivers’ visual comfort. Moreover, an EDVO-based evaluation index of degree of visual comfort at the tunnel portals has been established based on drivers’ ratings of visual discomfort and driving behaviors. Meanwhile, a quantitative relationship between the drivers’ EDVO and the illuminance transition when entering and exiting the tunnel is deduced and applied to calculate the stopping sight distance. The results show that the EDVO can make a good evaluation of visual comfort at tunnel entrances and exits where the degree of visual load can be arranged in a descending order as: daytime tunnel entrance > daytime tunnel exit > night-time tunnel exit > night-time tunnel entrance. It is noted that there exists strong positive correlations between the maximum pupil illuminance transition and the EDVO. It is suggested that the current stopping sight distance at the tunnel portals should be increased by 20–30 m to accommodate the general visual oscillation phenomenon.