## 2 Literature Review  
In the area of sudden fire disaster, researcher mainly focused on the evacuation problem over the last decade. [PERSON] (2018) investigated the dynamics of pedestrian evacuation with the influence of the fire spreading. A numerical model based on cellular automaton was proposed by [PERSON] (2018) to simulate the human behavior termed as flow with the stream, in emergency evacuation from a large smoke-filled compartment. Evacuation features from a terrace classroom were investigated by [PERSON] (2018) through simulations using both the models and experiments. A modified particle swarm optimization algorithm was proposed by [PERSON] (2018) to investigate the dynamic of pedestrian evacuation from a fire occurred in a public building--a supermarket with multiple exits and configurations of counters. [PERSON] (2018) proposed an approach which combines a multi-agent model with fuzzy logic to handle multiple behavior features of human-beings to simulate individual and group behavior during finding the safety egress. [PERSON] (2018) used agent-based simulator to analyze the evacuation processes under the surrounding that a sudden fire happening in a classroom with two exits. In traffic networks, the studies mainly focused on sudden fire disasters happening in tunnels. [PERSON] (2018) designed a simple and realistic model to calculate the hazards and influences of sudden fire disasters occurring in tunnel. An optimal smoke control strategy was developed and evaluated for an urban traffic link tunnel fire in Beijing center business district by [PERSON] (2018), and using critical velocity, minimal smoke spreading area and available safe evacuation time to evaluate the performance of proposed model. [PERSON] (2018) presented a model to simulate the effects of sudden fire disasters due to different vehicle types in a bi-directional road tunnel. A set of full-scale experiments was conducted by [PERSON] (2018) to study thermal and smoke control strategies using transverse ventilation system in a sloping urban traffic link tunnel. The results showed that the slower spread velocity of the smoke, the better effective of control strategy. Experimental studies on the smoke spread characteristics in a titled tunnel were carried out with a reduced-scale tunnel model by [PERSON] (2018). On the topic of fire disasters, the studied scenes are mainly within the building and urban traffic link tunnel. The research contexts are mainly distributed in dynamic diffusion mechanisms of fire, emergency evacuation models, and traffic behaviors under fire disasters. However, the studies on traffic flow characteristics in the traffic network under sudden fire disasters are limited in the state-of-the art research literatures. Traffic flow characteristics analysis is the foremost step in emergency traffic organization, such as evacuation route optimization, emergency vehicle dispatching, and traffic management and control in disaster-affected area. Therefore, this paper tries to analyze the traffic flow characteristics in urban high-density area under sudden fire disasters. In traffic flow analysis, the traditional traffic flow theory using the relationship between traffic flow parameters to explain the traffic flow characteristics is intuitive and easy to understand. However, if the road segment being saturated or supersaturated, it is difficult to describe the traffic flow characteristics accurately based on traditional traffic flow theory [11]. In urban high-density area, traffic aggregation and evacuation makes the traffic network congested under sudden fire disasters. The traffic flow characteristics are not suitable to describe by the normal two-dimensional relationship based on traditional traffic flow theory under this situation [25]. The catastrophe theory has the advantage in describing the mutation behavior of a system in natural or human society, such as biological variation, cell division, human behavior, congestion in public area. The processes of transition can be linear or nonlinear.