## 2 Road Traffic Situation Assessment Model
### Traffic Situation Assessment Indicator Analysis
#### 2.2.2 Construction of the Traffic Situation Prediction Indicators System  
Based on the principle of systemic and reliability of index design, the road traffic situation under emergency events is comprehensively evaluated from three aspects: road factors, natural factors and human factors. Firstly, the hierarchy is constructed by dividing the objective layer, criterion layer and indicator layer according to the decision objective, decision criterion and decision object. The constructed hierarchical structure model of road traffic situation impact under emergencies is shown in Figure 4, including three primary indicators of road impact, natural impact and human impact and ten secondary indicators such as road length, one-way road width and traffic lights. According to the hierarchical structure diagram, the assessment index system of the impact of the emergency event on the road traffic situation is constructed.  
(1) Road factors \(U_{1}\).  
The road is a basic element of road traffic. Its design should not only meet the function of vehicle traffic, but also minimize the urban road emergencies caused by the structural design. Road geometric feature, special road section and traffic facility are all factors that affect the emergencies, which all constitute road factors.  
Road geometric feature is the design part of the road. It mainly describes the road cross-section. Its indicators include road length, one-way road width, shoulder width, road median interval, etc. Special road sections in urban roads mainly refer to intersections, tunnels and other special structure sections included. They will cause visual errors to the vehicle drivers and thus increase the probability of emergencies. Setting traffic facilities is a way to manage the road. It will not only reduce the probability of road emergencies, but also help traffic management personnel to deal with emergencies quickly. A common traffic safety facility is the traffic light. This paper selects the common indicators of road length, one-way road width, traffic lights and intersections as road factor indicators.  
(2) Natural factors \(U_{2}\).  
The natural environment includes two aspects: climatic conditions and ecological landscape. For urban road networks, climatic conditions cover the weather changes in the road environment, such as rain, snowfall, freezing, fog, etc.; ecological landscape covers the natural landscape changes around the road, such as illumination, visibility, buildings, etc., and also includes factors such as landscape singularity and landscape visual attractiveness. These conditions thus affect the road traffic status. Its main influencing factors are rainfall intensity, temperature and visibility.  
(3) Human factors \(U_{3}\).  
Human factors are the key determinants of urban road emergencies. Human factors affect road traffic resulting in an increased probability of emergencies. It mainly includes road capacity, road construction and road enforcement. Road capacity is the ability of road  
Figure 4: The hierarchical model for urban road traffic situation assessment during emergencies.  
facilities to divert traffic flow. It determines the current traffic operation of the road. When it is easier to relieve the traffic pressure on the road, the probability of emergencies is lower. Road construction may occupy and close some lanes of the current roadway. This can interfere with the overall traffic flow, causing a reduction in traffic capacity. Road enforcement provides more detailed traffic management on the road. Once there is a tendency of emergencies, priority treatment becomes a focus, so as to quickly protect the road to restore road traffic and maintain the normal operation of the road. Its main influencing factors are road enforcement officers, traffic flow and road construction and maintenance.  
In summary, the indicator system for assessing the road traffic situation under emergencies is shown in Table 2. Finally, the set of constructed indicators is \(U=(U_{1},U_{2},U_{3})\).