## 4 Results and Analysis
### Analysis of Factors Influencing Tunnel Service State
In Figure 6a, the annual average daily traffic (F8) is ranked second in the SHAP ranking and fourth in the feature importance ranking for the structural section. Meanwhile, another human-related factor, the annual average daily truck traffic (F9), also ranks high (ninth and eighth, respectively). These findings are in line with the survey results for Chinese highway tunnels documented by [PERSON] et al. [7], who concluded that the human factor is an important contributor to tunnel defects during operation. Human factors impacted various sections in different ways. Civil section components such as pavements are in direct contact with vehicles and are thus subjected to wear and tear, fatigue, and impact from vehicles. In contrast, the structural section, which is designed to mainly bear the pressure from surrounding soil and water, is less likely to be in direct contact with normal running vehicles, and traffic volume might affect the service state of structural components more through damage to the tunnel structure resulting from tunnel operational accidents. [PERSON] et al. [27] analyzed 2703 tunnel traffic accidents in China from 2001 to 2019, finding that human factors were the primary causes of accidents and revealing that rear-end accidents and collisions with tunnel walls were the most common types of incidents, as shown in Figure 7. After a severe collision, a car may cause secondary disasters such as fires and explosions, posing a threat to the service state of tunnel structures. The sudden impacts resulting from secondary disasters such as explosions and fires can cause severe damage to a tunnel's structure, thus adversely affecting its functionality. Additionally, damage to structural elements in hidden locations (such as the contact surface between the lining and the surrounding rock) can be difficult to identify and repair after such disasters. Damage such as cracking provides a fast pathway for the transmission of aggressive ions, water, and oxygen in concrete, resulting in an accelerated rate of steel reinforcement corrosion and an increase in the corroded area within the elements (Figure 8). This process eventually accelerates the deterioration process of tunnel linings [28; 29; 30]. In addition, tunnels with high traffic volume are more likely to become targets of terrorists. These terrorist attacks can also produce cracks in such tunnels [31]. It is worth noting that the existing literature investigating the service states of pavements has generally adopted traffic volume as one of the most influential factors. However, research on the service states of tunnels' structural elements is currently more focused on the durability evolution of tunnel elements under the coupled effects of surrounding rock pressure and erosive environments [32; 33; 34]. There is still a lack of studies on the coupling mechanisms of impacts, explosions, fires, and other sudden effects generated by traffic accidents on the long-term service performance of tunnels. Furthermore, current design and evaluation methods for tunnel service performance can be improved to allow us to consider the possible impacts of sudden disasters on long-term degradation.