# Variable Speed Limit and Ramp Metering for Mixed Traffic Flows: A Review and Open Questions
###### Abstract  
The trend of increasing traffic demand is causing congestion on existing urban roads, including urban motorways, resulting in a decrease in Level of Service (LoS) and safety, and an increase in fuel consumption. Lack of space and non-compliance with cities' sustainable urban plans prevent the expansion of new transport infrastructure in some urban areas. To alleviate the aforementioned problems, appropriate solutions come from the domain of Intelligent Transportation Systems by implementing traffic control services. Those services include Variable Speed Limit (VSL) and Ramp Metering (RM) for urban motorways. VSL reduces the speed of incoming vehicles to a bottleneck area, and RM limits the inflow through on-ramps. In addition, with the increasing development of Autonomous Vehicles (AVs) and Connected AVs (CAVs), new opportunities for traffic control are emerging. VSL and RM can reduce traffic congestion on urban motorways, especially so in the case of mixed traffic flows where AVs and CAVs can fully comply with the control system output. Currently, there is no existing overview of control algorithms and applications for VSL and RM in mixed traffic flows. Therefore, we present a comprehensive survey of VSL and RM control algorithms including the most recent reinforcement learning-based approaches. Best practices for mixed traffic flow control are summarized and new viewpoints and future research directions are presented, including an overview of the currently open research questions. intelligent transportation systems; urban motorways; traffic control; ramp metering-variable speed limit; connected and autonomous vehicles; reinforcement learning +
Footnote †: journal: _Sci. single vehicle or a group of vehicles leads to a significant change in adjacent vehicles' speed during unstable traffic flow conditions. Such a change in speed is known as a shock wave, which propagates upstream [1]. This is a traveling disturbance and causes new significant irregularities in the upstream traffic flow. Such a shock wave is one of the the main causes of additional capacity drop and congestion on a critical segment of urban motorway due to unstable traffic flow. At the time of capacity drop, the vehicle flow measured immediately after the bottleneck area is significantly reduced relative to the maximum possible capacity of the observed segment [2; 3; 4]. Traffic control is one of the services from the domain of Intelligent Transportation Systems (ITS) that can alleviate congestion. The main Measures of Effectiveness (MoEs) used to assess the performance of traffic control systems are \(TT\), measured in units of time, e.g., (s), Total Travel Time (\(TTT\)), and the Total Time Spent (\(TTS\)) of all vehicles on the controlled segment of the urban motorway, both expressed in (veh \(\cdot\) h). LoS is another measure used to describe the traffic state labeled with letters from \(A\) to \(F\) and described by density and speed measurements, according to Highway capacity manual [5]. On urban motorways, the most commonly used traffic control systems are Variable Speed Limit (VSL) and Ramp Metering (RM). VSL controls the speed of the main traffic flow on the motorway and thus affects the dynamics of the traffic flow. By changing the speed limit on Variable Message Signs (VMS), VSL indirectly controls the inflow of vehicles on the controlled segment [6]. Thus, the VSL control system aims to achieve higher operational capacity of the existing traffic infrastructure on urban motorways without the need for additional traffic lanes. On the other hand, RM limits the number of vehicles entering the motorway from the controlled on-ramp. RM can be implemented as an open- or closed-loop control system. The main difference between open-loop and feedback systems is that the open-loop systems do not account the output of the system in the next control step, while feedback systems do. Classically used algorithms for RM use a feedback control loop such as the ALINEA algorithm. VSL and RM can be integrated and act in synergy.