# Study on the Driver Visual Workload of Bridge-Tunnel Groups on Mountains Expressways
## 5 Conclusions  
The planning and construction of mountain expressway bridge-tunnel groups are mainly affected by terrain and geological conditions. The light environment in the bridge-tunnel groups is also closely related to the safe operation of the mountain expressway. The inner space of the tunnel section of bridge-tunnel groups is enclosed, features a monotonous environment, and exhibits noticeable differences from the external environment. Under the condition of high-speed driving, the driver's visual field refresh frequency is high, and the visual distance is relatively narrow. The frequent abrupt change in lighting conditions in bridge-tunnel groups causes a great visual load on the drivers. The drivers keep shuttling between the outdoor environment and tunnels with huge differences in lighting, and their eyes keep experiencing the \"black and white hole effects\". When the distance between two continuous tunnels is too short, the drivers have to adjust promptly to light changes, i.e., the \"black and white hole effect\". If the drivers intend to pass through the bridge-tunnelgroups composed of multiple bridges and tunnels, they have to deal with frequent and repeated light-to-dark and dark-to-light transitions. These challenges will place a heavy visual load on the drivers, seriously affect their visual function, and make the bridge-tunnel group sections prone to accidents. Therefore, it is of great significance to study the visual changes of drivers and to reveal the mechanism of load changes. Through experiments, this study studies the visual load of drivers in mountain expressway bridge-tunnel groups based on fixation area, pupil diameter, _MTPA_, etc. The experimental results show that when driving in bridge-tunnel groups, the pupil diameter of the drivers in the first tunnel falls within the largest range [5.0 mm, 5.5 mm], and the driving load is also the largest. In other tunnels of bridge-tunnel groups, the pupil diameter and the number of fixation points of drivers rise with the increase of the length. For a single tunnel in a bridge-tunnel group, the driver pupil diameter at 0-50 m before the tunnel entrance and at the tunnel exit is proportional to the tunnel length. If the distance between adjacent tunnels is too large or too small, the driver will maintain a large pupil diameter, which will reduce driving comfort. The grade of the driver's visual load was evaluated based on _MTPA_. The results showed that a longer total length of bridge-tunnel groups, a larger number of tunnels, and a worse geometric index will all lead to more significant visual fluctuation and greater visual load. This study provides a reference for the analysis of driving behavior in bridge-tunnel groups and the design of tunnel lighting and shading facilities. The driving in bridge-tunnel groups is affected by many factors, which cannot be all incorporated into the current study, so there are some limitations to this study. Future improvement directions are listed below:  
1. In order to ensure driving safety in mountain highway bridge-tunnel groups, the participants of the experiments are all experienced drivers. The sample size is small, and the distribution is uneven. This study did not consider the influence of driver type on visual characteristics, and subsequent studies can further analyze the visual characteristics of drivers in terms of their gender, age, nationality, road familiarity, and driving style. 2. This study only uses two indicators, namely, fixation area and pupil diameter, to study driving behaviors in bridge-tunnel groups. Relevant studies on gaze time, gaze area division, and gaze transfer characteristics can be carried out in the future. Moreover, EEG, ECG, and myoelectric data can be obtained by physiological instruments to carry out a comprehensive analysis of the driver's driving load. 3.