Integrated Approach Combining Ramp Metering and Variable Speed Limits to Improve Motorway Performance

Abstract

Ramp metering (RM) and variable speed limits (VSLs) are two widely used intelligent transportation system (ITS) means to improve and manage motorway traffic. The former controls the flow of traffic into motorways from on-ramps, and the latter affects the speed of traffic on the motorway main line. An integrated approach to the prudent use of these two ITS measures can help to achieve optimal utilization of motorways. This study proposed a new method to integrate RM with VSL controllers to attain an efficient and equitable motorway system. The proposed method was used to combine a local and a coordinated RM strategy, namely ALINEA and Heuristic Ramp Metering Coordination (HERO), with VSLs. The method developed was assessed with a case study in Auckland, New Zealand. The analysis of the case study was based on a critical bottleneck section of Auckland Motorway by using the AIMSUN microsimulator. The outcome performance was assessed in relation to the efficiency and equity of the motorway system. The efficiency was measured by total vehicle travel time, average number and duration of stops, and emission levels; the equity of the motorway system was measured by the Gini coefficient. The main results were that the modified VSLs and the HERO + VSL control scenarios outperformed all other control scenarios in improving vehicular emissions, total travel time, and the equity measure, respectively. This outcome provides prospects for the developed method.