## 2 Road Traffic Situation Assessment Model
### Urban Road Monitoring Network
#### 2.1.2 Collaborative Monitoring Area Based on Emergencies  
The road traffic situation is closely related to node monitoring data in the network. Therefore, when conducting the traffic situation assessment for target events, a collaborative monitoring area should be determined according to the target events, and the characteristics of the road correlation between nodes should be fully considered. For the nodes distributed in the network, if these nodes can be classified and processed and the nodes associated with the target event can be arranged for collaborative monitoring, on the one hand, it is helpful to improve the accuracy of monitoring data, and on the other hand, it is beneficial to reduce the node communication energy consumption.  
The model adopts the event-triggering mechanism to build a road collaborative monitoring network, as shown in Figure 1. When a road sensor detects the target event, it transforms into the event source node and uploads the target event information. The central node searches for the road node that can monitor the target event cooperatively, determines the cooperative monitoring area based on the target event, and constructs the road cooperative monitoring network based on the target event.  
The correlation degree in a cooperative monitoring network refers to the degree of mutual contact between roads, that is, the number of connecting edges with the road as the endpoint. The greater the correlation degree of road nodes,  
Figure 1: A road cooperative monitoring network based on emergencies.  
impact of events on nodes and the closer the network. In the cooperative node network, the reciprocal of the node correlation degree is defined as the road correlation degree. The road correlation degree represents the basic probability that a node selects other nodes without considering road information. The urban road monitoring network takes the intersection as the main object, and the node correlation degree is 4. The above model is expanded into an undirected weighted network model, and its weight settings are shown in Figure 2.  
In the undirected node collaborative network \(G=\left\{V,E,W\right\}\), the node is expressed as \(V=\left\{v_{i}\mid i\in I,I=0,1,2,\ldots,N\right\}\), \(N\) is the number of nodes, where \(v_{0}\) is Master node, \(v_{a}\) is a monitoring node, \(a=1,\ldots,N\); there is an edge \(E=\left\{e_{ij}=\left(v_{i},v_{j}\right)\mid i,j\in I\right\}\subseteq V\times V\) between nodes \(v_{i}\) and \(v_{j}\). Combined with the network connectivity, the network correlation matrix \(Z=\left\{z_{ij}\right\}_{N\times N}\) can be constructed; the elements \(z_{ij}\) of the network correlation matrix \(Z\) can be expressed as Equation (1), where the element \(z_{ij}\) represents the connection between nodes \(v_{i}\) and \(v_{j}\), and \(n\) is the number of roads connected to its nodes.  
\[z_{ij}=\left\{\begin{array}{ll}0,&\left(v_{i},v_{j}\right)\in E\\ n(n\
eq 0),&\left(v_{i},v_{j}\right)\
otin E\end{array}\right. \tag{1}\]  
\(W=\left\{w_{ij}\right\}_{N\times N}\) is the road association matrix between nodes in the network. \(w_{ij}\) represents the degree of road association between nodes \(v_{i}\) and \(v_{j}\), and the degree of association follows a Gaussian distribution, where the element \(w_{ij}\) can be expressed as Equation (2).  
\[w_{ij}=\left\{\begin{array}{ll}0,&\text{Node $v_{i}$ and $v_{j}$ are not associated}\\ 1/n,&\text{Node $v_{i}$ and $v_{j}$ are associated}\end{array}\right. \tag{2}\]  
The degree of road association is related to the connection between the child nodes. Since the connection between the main node and the child nodes indicates that the main node has a communication link with the child nodes, the road association degree of the main node in the road association matrix is all 1; there is an association of road collaboration when node \(v_{i}\) is connected to \(v_{j}\) with a weight of \(1/n\), where n is the association degree of node \(v_{j}\); when there is no road association between nodes, the weight is 0.