# Variable Speed Limit and Ramp Metering for Mixed Traffic Flows: A Review and Open Questions
## 5 Conclusions  
The application of advanced control solutions in the domain of ITS enables further improvement of LoS on urban motorways. This paper reviews previous analyses of the impact of VSL and RM on traffic flow. The role of VSL and RM systems on urban motorways is described, and the operation of VSL controllers and RM control strategies is explained. Emphasis is placed on the description of VSL and RM control algorithms in mixed traffic flows. The impact of AVs and CAVs and the application of VSL and merging control approaches in mixed traffic flow scenarios are also analyzed. The results of recent papers on simulated traffic control systems for mixed traffic flows are also included in the review. The observed open research areas of applying VSL and merging control approaches in mixed traffic flows on urban motorways are as follows. Establishing a multi-agent VSL and merging system based on agent cooperation to maximize system performance. The multi-agent approach could be used to utilize the moving zone area under the VSL, calculate the speed limit, and use AVs and CAVs as actuators for VSL and merging control systems on urban motorways. All previous analyses are either based on the analysis of only one system (VSL or merging control). The application of VSL and merging control in mixed traffic scenarios could potentially smooth out speed harmonization, increase safety at merging areas on on-ramps and increase overall motorway capacity due to the lower headway gap that AVs and CAVs utilize, and communication characteristics that allow CAVs to utilize the motorway capacity more efficiently with cooperative LC. Another open area of research lies in the analysis of weather conditions on the behavior of AVs and CAVs, which can affect the performance of the traffic control system including the in-vehicle placed low level driving systems of (C)AVs. The lack of such analysis represents a shortcoming in current studies of mixed traffic flows with both VSL and merging control approaches. The conceptualization of the study was conducted by [PERSON], [PERSON], D.C. and E.I. The funding acquisition was conducted by [PERSON] and E.I. The writing of the original draft and preparation of the paper was conducted by [PERSON] and [PERSON]. All authors contributed to writing of the review and final editing. The supervision was conducted by E.I. Visualizations were conducted by F.V. All authors have read and agreed to the published version of the manuscript. This work has been partly supported by the University of Zagreb and Faculty of Transport and Traffic Sciences under the grants \"Control System of the Spatial-Temporal Variable Speed Limit in the Environment of Connected Vehicles\" and \"Innovative models and control strategies for intelligent mobility\", by the Croatian Science Foundation under the project IP-2020-02-5042, and by the European Regional Development Fund under the grant KK.01.1.1.01.0009 (DATACROSS). This research has also been carried out within the activities of the Centre of Research Excellence for Data Science and Cooperative Systems supported by the Ministry of Science and Education of the Republic of Croatia. The authors declare no conflict of interest. The funding institutions had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.