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.

Motorway Bottleneck Probability Estimation in Connected Vehicles Environment Using Speed Transition Matrices

“…The authors in [ 31 ] summarized different reinforcement learning methods to VSL on the motorways in mixed traffic flows that were used to improve the performance of the control system. The emphasis of research in [ 32 , 33 , 34 ] is using Connected Autonomous Vehicles (CAVs) as the mobile sensors that provide data for cooperative VSL, which is used as a speed control system on a motorway.…”

Influence of Spatial Placement of Variable Speed Limit Zones on Urban Motorway Traffic Control

“…In Equation 1, the difference of the sum of measured densities of all affected downstream sections from the previous control time step ρ j (t-1) and the sum of current densities ρ j (t) with respect to section i is added to the speed limit value from the previous control time step V i (t-1). Impact of density changes on the new speed limit V̅ i (t) can be set by the positive proportional gain K v : 1) and traffic is smooth with low interaction rates between vehicles [11]. Applying speed limits under critical densities decreases the slope of the fundamental flow-density diagram, thus, leading to reduced traffic flow (q VSL i ) from the VSL area.…”

“…At density values above the critical density (ρ>ρ c ) the traffic flow becomes unstable (right red side in the fundamental diagram, Figure 1) and vehicles interactions are more prominent. This may lead to shockwaves causing a chain reaction, resulting in a complete traffic jam on the motorway [11]. VSL moves the critical density to higher values in the flow-density diagram [12], thus, enabling more vehicles to be placed within the controlled motorway section without falling into unstable traffic state.…”

Lane Change Control Combined with Ramp Metering: A Strategy to Manage Delays at On-Ramp Merging Sections

“…is is especially true when the on-ramp and mainline demand are high. An overview of various RM algorithms is given by [5,12]. Variable Speed Limit (VSL) is a popular mainstream traffic flow control measure, which is used to regulate the inflow to critical sections and prevent high densities.…”