# Tunnel Traffic Evolution during Capacity Drop Based on High-Resolution Vehicle Trajectory Data
## 4 Analysis of Capacity Drop Based on Phase Transition-Theory
### Analysis of Capacity Drop Based on Traffic Flow  
This section uses vehicle speed's temporal and spatial characteristics as reference indicators, and the three main lanes' temporal and spatial trajectory diagrams are homogeneously abstracted. We then draw the three-phase traffic flow temporal and spatial characteristics of the main lanes (see Figure 3). [PERSON]'s empirical research shows that there are usually two main blocking patterns at a single bottleneck: the synchronized flow pattern (phase S) and the wide moving jam pattern (phase J). In [PERSON]'s theory, the phases J and S in congested traffic are observed outcomes in universal spatial-temporal features of real traffic data. Asynchronous flow appears at the bottleneck at the beginning of the congestion and propagates upstream of the bottleneck. The downstream front of the synchronized flow is often fixed at a bottleneck. In contrast, a wide-moving jam could occur spontaneously in this synchronous flow. A wide moving jam is meant to reflect the characteristic feature of the jam to propagate through any other state of traffic flow and any bottleneck while maintaining the velocity of the downstream jam front. The speed within the wide moving jam could be very low, and thus this pattern is also called stop-and-go traffic.  
There is a connection between the capacity characteristic point and the phase change boundary of the three-phase traffic flow.  
Figure 3: Analysis of tunnel capacity drop based on three-phase traffic flow.  
1. F->S: Free flow changes to synchronous flow. Traffic crash occurs with the spontaneous start and propagation of the synchronous flow (the upstream boundary of the synchronous flow propagates upstream, while the downstream boundary is fixed at the bottleneck). According to the black dotted arrow in the distance-time diagrams in the first row of subfigures in Figure 3, the vehicle begins to slow down when approaching the upstream interface, and the bottleneck capacity does not change significantly at this time.
2. S->J: Synchronous flow changes to jam flow. Some narrow moving jams (narrow moving jams) appeared in this state, and then they evolved into wide moving jams (J appeared). At this time, the capacity has been significantly reduced.  
Combined with the characteristic points of capacity drop in the temporal and spatial diagram of the vehicle trajectory in the figure, it is found that the vehicle speed decreases significantly when the traffic flow is approaching the interface. It causes drivers to take a more considerable distance between vehicles, leading to the phenomenon of \"traffic capacity drop\" at the macro level. From the flow-density diagrams in the subfigures of Figure 3, it can be found that the traffic capacity drop points are all near the synchronous flow, and vehicles exhibit a significant speed reduction. The capacity decreased rapidly when the synchronous flow transitioned to the wide moving jam flow. In the synchronous flow state, the vehicle speed decreases while the density increases, and the traffic capacity does not change significantly. The synchronous flow is more like the accumulation process of disturbance. When the disturbance is sufficient, the jam flow occurs.