# Variable Speed Limit and Ramp Metering for Mixed Traffic Flows: A Review and Open Questions
## 4 Discussion  
The focus of the paper was on the description of VSL and merging controllers and proposed control algorithms in mixed traffic flow scenarios. We have identified several potential research directions to address the limitations of existing methods. The limitations are the need for an accurately estimated fundamental diagram and the implementation of cooperative VSL and merging control system in mixed traffic flows. The main drawback of the efficiency of classical VSL algorithms in mixed traffic flows is their inability to adapt their control policy to a new traffic situation, in which they perform sub-optimally. In the last five years, according to Table 2, there has been an increase in the number of studies focused on improving and proposing new VSL control algorithms for mixed traffic flows. The drawback of both VSL and merging control approaches in mixed traffic flows is the assumption that the communication network is error free and that there is no delay or loss of information sent to the vehicles. Although AVs and CAVs are still at an early stage of development, a comprehensive study that includes both the control approach and the communication network influence analysis is yet to be made. In [80], an analysis of mixed traffic flow was performed with CA model on a road segment divided into 3000 cells. The length of each vehicle was set to 1 cell and the maximum desired speed was set to 5 cells/s. The model could be improved by adjusting the vehicle length and maximum desired speed, which would lead to more realistic scenarios and results by keeping the definition of cell lengths in mind. A successful DRL model for DVSL control, where dynamic and distinct speed limits can be imposed between lanes, was implemented in [84]. Four learning reward scenarios were tested. Even though the results are promising, a safety analysis on incidents and crash risk or collision rate should also be considered. Due to differential speed limits, LCs could potentially lead to unwanted incidents in real-world scenarios. In [99], it was stated that CIDM system performance under different proportions of AVs for motorway merging should consider safety aspect for low safe time gaps. For mixed traffic flows, HDVs, unlike AVs, cannot maintain low safe time gaps, where HDVs could potentially have no space to perform safe LC when needed. In [85], the authors successfully used GA to solve the MPC formulated problem of VSL and LC. Using the CAVs' information about their average speed, density, and flow rate in each cell, GA repeatedly generates a population of individual solutions. These individual solutions are evaluated based on the objective function value in the next prediction horizon according to the modified multi-class CTM with these collected data. Most researches on merging control approaches in mixed traffic flows disregard the classical RM approaches. Instead of the classical RM control, which uses a traffic signal with real-time traffic data to determine the rate at which vehicles should enter the motorway, merging control approaches are proposed to resolve interactions and conflicts between vehicles in merging areas [95; 98; 99; 100]. These strategies can be achieved by assuming that all vehicles are CAVs. One of the major shortcomings of VSL and RM control algorithms for mixed traffic flows is that they can become obsolete at very high penetration rates. The advantages of AVs and CAVs in future mixed traffic flows at any penetration rate are pronounced in improving the macroscopic traffic parameters of urban motorways reducing the need for separate control. The major shortcoming in the application of RL is the lack of a precise explanation of the learning process. The question of why the agent learned a particular strategy of action could not be answered accurately. Another issue that needs to be addressed is the cooperation of VSL and merging controllers in mixed traffic flows by applying control algorithms to improve MoEs and LoS.