# The Car-Following Model and Its Applications in the V2X Environment: A Historical Review
## 4 Discussion
### The Existing Shortcomings  
Idealized Assumptions in Modeling Car-following Behavior  
Among the previous works, the traffic flow in most of the models was set as uniform flow, which means the penetration of V2X technology in the system is 100%. However, as is well known, the development of anything is not achieved overnight, and the V2X technologies are no exception. As a typical kind of advanced informatization technology, the development of V2X technologies certainly is a process by which the equipment will gradually become common, functions will become gradually comprehensive, and performance will continuously improve. In this development process of V2X technology, the penetration of connected vehicles with different equipment will certainly increase from low to high. Considering the unbalanced economical development in different regions, the penetration may never reach 100%. However, the impacts of the penetration of connected vehicles were not considered in most of the previous studies. Incomplete Considerations in Modeling Car-following Behavior  
Along with the penetration of connected vehicles, the attributes of drivers and vehicles were also not incorporated in most of the previous works. The drivers' car-following behavior is essentially the implementation of their subjective driving will, which is time-varying and easily affected by factors internal and external. There are significant differences in the physical and dynamic characteristics of various vehicles, which could affect the driving behavior, such as the car-following one to a considerable extent. In the V2X environment, the driver can obtain massive amounts of traffic information, including about the attributes of drivers and vehicles in the system. However, there are few works incorporating these attributes when modeling car-following behavior in the V2X environment. Absence of Global Perspective in Modeling the Car-following Behavior  
In the V2X environment, the informatization degree of transportation systems has been significantly improved, which theoretically enables the drivers to obtain massive amounts of information of all units in the system. In the previous works, the impacts of various sources of information, such as headway, relative velocity, and acceleration, on car-following behavior were discussed, and there is a small number of works that explored the impacts of information about vehicles in the adjacent lanes. However, the aforementioned works only incorporated one or several kinds of information in isolation, and research incorporating information about all relevant units in the system is still absent. Scarcity of General Modeling Method/Framework  
The models constructed based on various theories in the previous studies are able to describe the car-following behavior under specific conditions. However, due to the differences in basic theory and hypothetical conditions, the generality of these models is poor, which is mainly reflected in two aspects. On the one hand, the performance of these models to describe the car-following behavior beyond the initial hypothetical conditions is upset, meaning these models can only be employed in the environment within their hypothetical conditions. These conditions are a limitation. On the other hand, it is hard to effectively compare these models to discuss the differences in car-following behavior affected by various sources of information. General models or modeling frameworks, which are suitable for all development phases of V2X technology and that can describe the car-following behavior under various conditions, along with the corresponding evaluating methods for cross comparison, should attract more attention in future works.