# Experimental Analysis of Driver Visual Characteristics in Urban Tunnels
## 1 Introduction  
To ease urban traffic congestion and reduce traffic delays, massive networks of urban expressway networks have been built in big cities. A tunnel under an urban expressway does not affect the urban landscape, occupies a small area, and has a positive effect on reducing the land use of urban roads, shortening the driving mileage, and facilitating urban traffic. Underpass tunnel interchanges are mostly adopted in downtown areas. However, the urban tunnel structure is closed, its internal environment is monotonous, and the light intensity, temperature, and humidity of the inside and outside of the tunnel are quite different [1, 2, 3]. Therefore, the driver's physiological and psychological state will change when driving in the tunnel environment, which will affect not only the driver's correct perception of the surrounding environment information but also the driver's decision-making and manipulation behavior [4, 5]. In recent years, the number of traffic accidents in urban tunnels shows a trend of increasing. [PERSON] et al. counted a total of 3976 traffic accidents occurring on the first ring road in Hefei in 2015, of which 325 occurred in urban underpass tunnels, accounting for 8.2% [6, 7]. Due to dense urban population and largetraffic flow, tunnel section, as the bottleneck of an urban expressway, is not only prone to accidents but also difficult to deal with after accidents. Therefore, it is extremely important to study the traffic safety of urban tunnels. However, existing tunnel traffic safety research mainly focuses on expressway tunnels, and the research related to the traffic safety of urban tunnels is relatively scarce, but whether expressway tunnel traffic safety-related research conclusions are applicable to urban tunnels still needs to be further verified [8; 9; 10]. At present, the traffic capacity of the entrance and exit sections of urban tunnels is low, and the phenomenon of drivers changing lanes at will to overtake is serious. At the same time, the speed limit of urban expressway tunnels is generally 60 km/h or above, and some sections even reach 80 km/h (such as the Nanjing Yangtze River tunnel speed limit of 80 km/h), which is consistent with the speed limit of expressway tunnels. It is necessary to carry out special research on traffic safety of urban expressway tunnels, which can further improve the theoretical system of tunnel traffic safety research theoretically and practically, improve the driving efficiency of urban tunnel sections.  
Existing research confirms the significant influence of the tunnel environment on drivers' eye movement parameters and establishes a relationship between eye movement parameters and tunnel parameters such as illumination, driving speed, and visual induction facilities [11]. There are three basic forms of eye movement: winking, saccade, and gaze. Fixation indicators mainly include fixation area, fixation time, pupil diameter (or area), whereas saccade indicators mainly include saccade time, saccade speed, and saccade amplitude, and blinking indicators commonly used include blink duration and blink frequency [12]. This paper carried out a driver visual characteristics experiment for an urban expressway tunnel. It was based on experimental data analysis of urban tunnel-driving pupil diameter, scanning time, and interest area. Then, the time series clustering method was used to cluster the visual characteristic parameters of drivers, and the clustering results were analyzed so as to provide a reference for the study of urban tunnel traffic safety.