Motorway Management Techniques
A O'Brien, AECOM, UK
The role of different mechanisms to management motorways, and the interaction between them.
The provision of ITS equipment for management of motorways has become commonplace throughout Europe. Installations typically comprise measures to manage mainline traffic streams in an attempt to avoid flow breakdown, with additional measures to enhance capacity or restrict excessive traffic volumes from joining the mainline under congested or near-congested conditions.
Over the past 15 years, a general consensus has emerged regarding the role and function of function of Ramp Metering and Access Control on motorways, and has supported the deployment of numerous systems throughout the world. Likewise, the benefits of Hard Shoulder Running can be easily established. For Variable Speed Limits, the discussion is somewhat less certain. US deployments have focused on objectives to improve safety, whereas UK systems have a stated objective to improve capacity as well as safety. A research review highlights evidence to suggest that it can be extremely difficult to generate journey time benefits from a VSL system, and as such key benefits will comprise safety improvements resulting from a general reduction in stop-start driving. This assertion is borne out by results of the evaluation of the M25 in London, which highlighted quite limited journey time benefits but a notable safety improvement.
This evidence seems to contradict the general consensus that reducing the incidence of flow breakdown can protect carrying capacity of a motorway, given that flow breakdown itself can reduce carrying capacity locally by up to 15%. As such, removing such occurrences is key to avoiding the onset of such bottlenecks. In other words ? if avoiding flow breakdown through VSL installations is theoretically sound, why have such benefits not been observed in practice.
The current paper examines the impact of VSL, Ramp Metering and Hard Shoulder Running as individual strategies, and progresses to understand how they interact with each other. The paper uses Microsimulation techniques to understand vehicle behaviour in various environments, and the impact of various combinations of strategies. Nevertheless, the objective here is not to understand the preferred strategy for a particular situation, but instead to understand why such responses are occurring. This informs a general understanding of the function of the different systems, and how they influence the flow of the traffic stream.
The exercise examines a combination of physical road layouts, including motorways with no merging/diverging, motorways with merging slip roads with and without a lane gain, and a series of different mainline and slip road traffic flows. A number of conclusions are becoming evident from the study:
- That slip road flows have a disproportionate effect on mainline delay. As such, ramp metering is highly effective in reducing delay within the parameters set out in the Highways Agency Guidance note.
- Variable Speed limits are a highly effective means for dissipating flow breakdown, as they effectively remove ?bulges? in vehicle concentration by holding back the traffic stream behind the emerging bottleneck.
- Nevertheless, slip roads operate on the basis that the headway between vehicles on the mainline are variable, and that gaps can be found in high volume traffic streams as a result of this variability. Variable Speed Limits work to reduce this variability as they maximise the flow on the mainline.
- As such, with Variable Speed Limits, the ability of the merging traffic to join is inhibited. As gaps reduce, impacts of delay due to merging traffic increase disproportionately, and hence the potential for flow breakdown at a merge can be exacerbated. Under such high flow conditions, the ability of the Variable Speed Limits to dissipate such flow is reduced.
- The research also highlights that under conditions where a lane-gain merge is provided, that the net increase in flow through a merge is matched by an increase in capacity, and hence the concept of Variable Speed Limits can work very well, although under such conditions ramp metering becomes less warranted.
These conclusions therefore start to allow a clearer understanding of the role of each of these systems, and the conditions under which they may be most appropriate. This research will inform the development of a Network Management Strategy for the National Roads Authority (Rep. of Ireland) which will be integrated into the broader infrastructure programme over the period to 2025.
Association for European Transport