## 3 Causes and Characteristics of Traffic Interruptions in Highways to Tibet
### Characteristics of Traffic Interruptions
#### 3.2.3 Traffic Interruption Severity  
The severity of traffic interruption refers to the effects of interruptions on road traffic capacity, denoted by [21]  
\[s=t\times l \tag{1}\]  
where \(\mathbf{s}\) is the set of the severity of traffic interruptions, i.e., \(s=s(i),i=1,2,\ldots,n;s(i)\) is the severity of traffic interruption induced by the \(i\)th cause; \(t\) is the set of traffic interruption durations; \(l\) is the set of traffic interruption mileage. The index values for the severity of traffic interruptions along each highway to Tibet are calculated, and their distribution is compared. The Sichuan-Tibet Highway experiences significantly higher mean traffic interruption severity compared to the other highways, while the Xinjiang-Tibet Highway exhibits the most dispersed data (Figure 8). Compared with the Yunnan-Tibet Highway and Xinjiang-Tibet Highway, the severity of traffic interruptions of the Qinghai-Tibet Highway is higher, and its values are relatively discrete. The highways ranked from highest to lowest mean interruption severity are as  
Figure 6: Cumulative distribution curves of traffic interruption mileage. Figure 7: Boxplot of traffic interruption mileage. follows: Sichuan-Tibet Highway (77.10 km/h), Xinjiang-Tibet Highway (18.81 km/h), Qinghai-Tibet Highway (12.00 km/h), and Yunnan-Tibet Highway (0.33 km/h). The climate, topography, and geology along the highways to Tibet are notably complex and variable. Consequently, traffic interruptions along these highways exhibit distinct causes and characteristics. On the Qinghai-Tibet Highway, traffic accidents are the primary factor causing interruptions, followed by snow disasters. Occasionally, the Qinghai-Tibet Highway experiences traffic interruptions with extended duration and mileage. The duration of these interruptions can extend up to 29 h, while the distance affected reaches 20 km, indicating a high level of severity. The Qinghai-Tibet Highway, a critical transportation route, handles more than 85% of the materials entering Tibet and over 90% of those leaving. It boasts an average altitude exceeding 4000 m and an average annual temperature below freezing. Unlike other highways, the Qinghai-Tibet Highway traverses sections with frozen soil areas. The thaw settlement of frozen soil, along with environmental extremes, leads to pavement subsidence, instability, and cracking, which deteriorates driving conditions. In the case of low-temperature hypoxia and single-lane traffic, the dynamic performance of the truck is reduced. At the same time, drivers who work under such conditions for a long time may feel tired and slow to react, resulting in traffic accidents and blockages. Single-point congestion is easy to form a chain reaction--when a section of the road obstacles or traffic accidents (such as collapse and traffic accidents mentioned in Figure 1), it can quickly lead to serious local congestion. This is especially problematic on road sections where only one lane is available in both directions, blockage in any direction may lead to a significant reduction in the operational efficiency of the entire route. The Sichuan-Tibet Highway is located in a typical alpine and gorge region, characterized by complex geological conditions along both its northern and southern lines. The terrain is marked by loose geology, sparse vegetation, and a high susceptibility to collapses. Heavy summer rainfall triggers debris flows, while winter brings significant snow accumulation. Additionally, factors such as low technical standards, great linear changes, limited detour options within sections, and poor disaster resilience contribute to frequent traffic interruptions along the Sichuan-Tibet Highway. This route experiences the highest severity of traffic interruptions. Affected by geological and meteorological disasters, traffic interruptions with extended duration and mileage are common.