# Study on the Driver Visual Workload of Bridge-Tunnel Groups on Mountains Expressways
## 1 Introduction
Due to the complex terrain of mountainous expressways, bridge-tunnel groups sections with long tunnels, and steep downhill sections with large altitude differences are prone to severe accidents. In this sense, driving safety and comfort in such sections have been a concern [9]. Tunnels are typical road sections with poor visual environments, and driving in tunnels is relatively risky [10]. At present, transportation standards and regulations in China lack specific provisions regarding the design of tunnel group alignments [11; 12; 13]. Tunnel groups on mountainous expressways feature long tunnel distances, numerical tunnels, short tunnel spacings, and significant variations in lighting conditions inside and outside tunnels. Collisions are the main traffic accidents in bridge-tunnel groups, and high accidents occur at the entrance and exit areas. Rapid alternations in illuminance at entrances and exits and excessive vehicle speeds are the main causes of accidents [14; 15]. The traffic environment of tunnel groups is more complex than individual tunnels. The interactions between tunnels increase the driving risks. Research on tunnel group design and operational management is needed [16]. Due to unique locations and complex external environments, bridge sections, typically have higher collision severity, mostly from single-vehicle collisions, than normal sections of highways, tunnels, and service areas [17]. Traffic risks intensify under poor visibility and adverse weather conditions [18]. The upstream and downstream of the river-crossing bridge are considered high-risk corridors in freeway bridge sections [19]. Affected by unique and complex terrain, increasing sections on mountainous expressways constitute long tunnels, bridge-tunnel groups, and various interchanges. The proximity between these structures or the difficulty in identifying traffic signs leads to risky behaviors such as short-distance lane changes and rear-end collisions [20]. Roads of mountainous highway bridge-tunnel groups are narrow, with high curvature and steep slopes. [PERSON] et al. have established traffic operational risk classification criteria for bridge-tunnel groups based on the cumulative frequency curve of average risk indicators [21]. The bridge-tunnel-interchange group sections on mountainous expressways operate in completely enclosed environments, making traffic accident rescue operations difficult, and these segments have a widespread and long-lasting impact [22]. During vehicle operation,drivers need to control their vehicles to perform lane changes, following maneuvers, overtaking maneuvers, etc. This process requires coordination among drivers, vehicles, roads, and the environment in which drivers play a crucial role [23]. Researchers have increasingly explored the integration of physiological, psychological, and driving behavior studies, with eye trackers and physiological monitors widely used in investigating driver behavior characteristics. Driver behavior can be divided into three stages: visual perception, judgment and decision-making, and driving operation. Specifically, road perception and visual cognition are important human factors for driving comfort and safety [24]. Drivers mainly rely on their vision to obtain information during driving. In this sense, driver vision is directly responsible for traffic safety [25; 26]. Eye-tracking research began in the 1970s and has gained further development [27]. Simulation modeling, driving simulation, and on-road vehicle experiments are research methods for studying the eye movement behavior of drivers traveling in highway tunnels. Through these experiments, drivers' eye movement data are collected to analyze various indicators such as pupil diameter size, standardized pupil diameter, average speed of pupil area change, maximum transient velocity of pupil area (\(MTPA\)), pupil constriction, etc. [28; 29]. [PERSON] et al. found the eye-gaze behavior of drivers can be used to evaluate driving safety and comfort in tunnel groups [16]. [PERSON] et al.