Mainstream traffic flow control on motoways

Abstract

The continuously increasing daily trac congestions on motorway networks around the world call for innovative control measures that would drastically improve the current trac conditions. Mainstream trac ow control (MTFC) is proposed as a novel and ecient motorway trac management tool, and its possible implementation and principal impact on trac ow eciency is analysed. Variable speed limits (VSLs), suitably operated and enforced, are considered as one (out of several possible) way(s) for MTFC realisation, either as a standalone measure or in combination with ramp metering. A quantitative model of the VSL impact on aggregate trac ow behaviour on motorways is proposed and allows for VSLs to be incorporated in a macroscopic second-order trac ow model as an additional control component. The integrated motorway network trac control problem involving ramp metering and VSLs is formulated as a constrained discrete-time optimal control problem and is solved eciently even for large-scale networks by a suitable feasible direction algorithm. An illustrative example of a hypothetical motorway stretch as well as a large-scale motorway ring-road are investigated under dierent control scenarios using the optimal

control approach. It is shown that trac ow eciency can be substantially improved when MTFC via VSLs is used with or without integration with coordinated ramp metering actions. Since sophisticated optimal control methods may face diculties in practical eld implementations, three simple controllers for local feedback MTFC on motorways, enabled via VSLs, are proposed in this thesis. All feedback controllers rely only on readily available real-time measurements (no on-line model usage and no demand predictions are needed), take into account a number of practical and safety restrictions, and are therefore robust and suitable for eld implementations. The controllers are evaluated in simulation and compared with the optimal control results for a hypothetical motorway stretch. Despite their simplicity, the results show that the feedback controllers exhibit a satisfactory control behaviour and, indeed, approach the optimal control results for a number of investigated scenarios. Recommendations for the operation of the feedback controllers are given.