# The Car-Following Model and Its Applications in the V2X Environment: A Historical Review
## 4 Discussion  
Safety, congestion, and pollution, etc., are very common in transportation systems around the world. With the development of the economy and urbanization, this present situation is likely to become worse. In recent years, the relevant technologies represented by the V2X have been regarded as an effective means to solve or alleviate the above problems. The penetration of V2X technologies has made the present-day traffic environment different from that of the past, and thus significantly influenced the car-following behavior and traffic flow. Due to these factors, the car-following model and its applications have been one of the hot and frontline topics in the field of traffic flow theory. In this paper, we first reviewed the development process of traditional car-following models and the research on the impacts of the V2X environment. In the V2X environment, the most obvious changes caused by the penetration of relevant technologies are the informatization level of units in the system, including the driver-vehicle unit, which are significantly improved. Thus, the driver-vehicle unit can obtain massive amounts of traffic information. Considering this, the information in the V2X environment was divided into the forward and backward information based the variable attention of the driver in this paper. Based on this, research on the impact of various sources of information on car-following behavior along with the corresponding models were reviewed according to their development process. The results of these works reveal that the characteristics of car-following behavior and traffic flow affected by various sources of information in the V2X environment are different from the past, which is mainly reflected at:  
* The micro-level. The car-following process of vehicle(s) has been significantly optimized. In other words, the motion state of the vehicle(s) in all three stages (i.e., the normal car-following stage, the start-accelerating stage, and the braking-stop stage) has been improved. Specifically, the motion state of the vehicle(s) in the normal car-following stage is steadier and approximates the optimal state within less deviation. In the other two stages, the starting/braking process needs less time, and the safety, as well as the comfort of these processes, have been improved. Meanwhile, the headway and velocity of vehicle(s) during the car-following process are different from the past environment without V2X technology. * The macro-level. Generally, the traffic flow can operate in a better state. Specifically, for the disturbance with the same scale, it can be absorbed by the traffic flow in the V2X environment in less time, and the deviation between the current state and the optimized state of traffic flow in the environment when resisting the disturbance is smaller than that in the traditional environment. For the disturbance with different scales, the traffic flow in the V2X environment can maintain a steady-state when encountering larger disturbance. When operating at the steady-state, the efficiency of the traffic flow of the road segment or intersection in the V2X environment is much higher than that in the previous environment, which is the expression of the aforementioned optimization at the micro-level. When the traffic flow is operating out of the steady-state, the deviation can be kept in a smaller range. It is more difficult for the traffic flow to reach the completely blocked state, and the traffic flow will show the different propagating and evolving characteristics of the density wave. The impacts of various sources of information on car-following behavior were explored and the corresponding car-following models were established in the studies reviewed in this paper.