# Research on a Collaborative Control Strategy of an Urban Expressway Merging Bottleneck Area
## 1 Introduction  
As the highest-grade road in the urban road network system, urban expressways bear the function of the main skeleton of the road system. Compared with highways, expressways have a more concentrated distribution of entrances and exits, and adjacent entrances and exits are more closely connected with urban roads, resulting in more traffic conflicts on expressways than on highways [1]. Compared with urban arterial roads, expressways often bear more traffic demand due to their high design standards and closed traffic environment, resulting in a wider range of traffic bottlenecks and a faster diffusion rate. The traffic control methods for expressways are mostly based on traffic flow models, and the simulation of expressway traffic state is realized by establishing models that conform to the changing expressway traffic flow trend. Compared with the microscopic and mesoscopic models, macroscopic models [2] focus on the aggregation behavior of traffic flow and involve three main parameters of interest: traffic density, speed, and flow. Macroscopic models define the traffic flow as a continuous fluid, which focuses on the continuous medium behavior, and simplifies the individual dynamic behavior between adjacent particles, such as lane changes and car-following. The characteristics of fewer parameters and fast calculation make it have great potential in the study of traffic flow management and control. With the development of the macroscopic continuous model LWR [3, 4, 5], classical macroscopic traffic flow models such as CTM [6] and METANET [7] have evolved. With the understanding of traffic flow, scholars have proposed improvements to the basic CTM to make it more in line with the traffic flow state under different scenarios. The improved CTM is applied to unmanned vehicles' car-following model [8], vehiclerouting [9], lane change overtaking [10], traffic congestion control strategy [11], freeway ramp control [12], and so on. The existing research on the improvement of macro traffic flow has shown little consideration of the complex traffic characteristics of the expressway merging bottleneck, which means the macro traffic flow model may have low simulation accuracy when applied to the related research of expressway merging bottleneck. On the other hand, variable speed limit control of urban expressways has also attracted a lot of attention. Some scholars have studied the influence of variable speed limit control on traffic flow parameters. The research direction is the impact of variable speed limit control on traffic safety, road traffic performance, and traffic parameters. In terms of safety, this work [13] used the driving experiment method to prove that variable speed limit control can be used to improve traffic safety levels by collecting driver feedback. The research [14] used the accident risk model similar to [15] to evaluate the state of the road network and further proved that the variable speed limit control could improve the safety factor and road operation efficiency. In terms of traffic performance and traffic parameters, research by [PERSON] [15] found that variable speed limit control can maintain a certain equilibrium speed in each segment, thereby improving the speed inhomogeneity. The article [16] demonstrated in field tests that variable speed limit control can change the shape of the traffic flow fundamental diagram to control traffic flow at a high-density level before a sudden change, thereby improving freeway traffic performance during peak hours and easing traffic congestion. The work [17] used the traffic accident risk model to evaluate the effectiveness of variable speed limit control. Due to the contradiction between traffic supply and demand, independent control methods are unable to adapt to the changing traffic flow characteristics in complex situations.