Towards an Efficient and Equitable Motorway System using Ramp Metering and Variable Speed Limits

Abstract

Ramp metering (RM) and variable speed limits (VSL) are two important ITS tools aimed at improving the performance of motorway systems. The former improves the traffic flow conditions on motorway by limiting the inflow of vehicles from on-ramps; while the latter harmonizes the traffic flow on a motorway mainline by minimizing the variation in speed of vehicles obeying the VSL display. The main goal of this research is to improve the efficiency and equity performance of motorway systems using RM and VSL control measures. A number of RM and VSL control algorithms are proposed in this thesis to achieve this goal. They are assessed for a critical bottleneck section of Auckland Motorway using micro-simulation software AIMSUN. First of all, two existing RM algorithms are assessed namely: ALINEA and HERO. HERO outperformed ALINEA in terms of efficiency and equity benefits. Then, a logic tree based VSL control algorithm is also assessed. The results revealed that the existing logic tree based VSL algorithm cannot improve significantly mobility performance of motorway systems and is rather rough for the speed control. Two alternative VSL control algorithms are proposed namely: a modified logic tree based controller and a fuzzy logic based controller. Both utilize optimized control logics and detector-controller configurations. The proposed algorithms outperformed the existing one in terms of their mobility benefits. In the next step, an integrated RM and VSL control method is proposed to preserve the traffic flow at bottlenecks close to their capacity and avoid excessive delays at on-ramps. The integrated method shows prospect to improve further efficiency and equity performance of motorway systems than operating the RM and VSL controllers independently. Finally, the relationship between efficiency and equity is investigated using a modified HERO strategy; this is aimed at achieving a balance between efficiency and equity gains from RM control. A combined index is proposed combining Gini coefficient and total travel time into one index, which can serve as an objective function to solve the bi-objective control design problem.