# Study on the Driver Visual Workload of Bridge-Tunnel Groups on Mountains Expressways
## 1 Introduction  
With the continuous improvement of the expressway network in China, the mountainous expressways in the central and western regions have been extensively constructed, with a rapid increase in the mileage of bridges and tunnels. The accessibility of bridges and tunnels in mountainous areas allows for short-distance crossing of complex terrains, traffic efficiency improvement, and fragile ecological environmental protection of the mountains. In China, the \"Technical Standards for Highway Engineering\" (JTG B01-2014 [1]) defines an extra-large bridge as a bridge with a total span length greater than 1000 m or a single-span length greater than 150 m. Bridges with total span lengths ranging from 100 m to 1000 m or with a single-span length range of 40-150 m are classified as large bridges. Similarly, tunnels can also be categorized by length according to the standard as follows: extra-long tunnels with a length larger than 3000 m, long tunnels with a length range of 1000-3000 m, medium tunnels with lengths between 500 and 1000 m, and short tunnels with a length range within 500 m.  
By 2020, China had 912,800 highway bridges with a length of 66,285,500 m, an increase of 34,500 bridges and 5,651,000 m over the previous year. Among them, 6444 extra-large bridges with a total length of 11,629,700 m and 119,935 large bridges with a total length of 32,777,700 m were built. The country also had 21,316 highway tunnels with a length of 21,999,300 m, an increase of 2249 tunnels and 3,032,700 m over the previous year. A total of 1394 extra-long tunnels with a total length of 6,235,500 m and 5541 long tunnels with a total length of 9,633,200 m were built [2]. The adjacent tunnels and bridges form tunnel or bridge-tunnel groups. With the expansion of long tunnels, extra-long tunnels, extra-largebridges, and bridge-tunnel groups, the proportion of tunnel and bridge sections in the total highway mileage continues to increase, and their proportion may even exceed that of ordinary road sections. For example, the Chengkai Expressway in Chongqing, China, measures a total length of 128 km, and bridges and tunnels occupy 78% of the total length. In the case of the Yakang Expressway, in Sichuan Province, China, the study road, bridges, and tunnels account for 82% of the total length (135 km). The operational safety in mountainous bridge-tunnel groups is closely related to factors such as terrain, climate, road alignment, the combination of special structures, and driver behavior, making them potentially high-risk sections challenging safety and efficiency [3; 4]. [PERSON] and [PERSON] studied traffic accidents in Norwegian road tunnels and found that the accident rate at tunnel entrances was the largest, and the severity of accidents inside tunnels was higher than on open roads [5]. [PERSON] and [PERSON]'s research indicated that the rates of severe accidents and costs inside tunnels were generally higher than those on corresponding highways in Italy [6]. With the growing construction of bridge-tunnel groups on the mountainous expressways in China, accidents under such road conditions have received increasing attention. The traffic accidents in the bridge-tunnel groups in the Chongqing section of the Yuxiang Expressway accounted for approximately 23.2% of the aggregate road accidents [7]. Some serious accidents have occurred in the bridge-tunnel groups on expressways in western China. On 10 August 2017, a large bus collided with the entrance of the Qinling No. 1 Tunnel when traveling on a bridge-tunnel section, resulting in 36 deaths and 13 injuries [8]. Tunnels are considered bottleneck sections of expressways and exhibit characteristics such as confinement, inaccessibility, and difficulties in communication and rescue. Given that tunnel groups consist of adjacent tunnels, drivers experience rapid transitions between \"dark adaptation\" and \"light adaptation\" when they continually enter and exit tunnels, making tunnel groups sensitive areas on roads.