Unlimited access to a motorway network can, in overloaded conditions, cause a loss of capacity. Ramp metering (signals on slip roads to control access to the motorway) can help avoid this loss of capacity. The design of ramp metering strategies has several features in common with the design of access control mechanisms in communication networks.Inspired by models and rate control mechanisms developed for Internet congestion control, we propose a Brownian network model as an approximate model for a controlled motorway and consider it operating under a proportionally fair ramp metering policy. We present an analysis of the performance of this model. AMS subject classification (MSC2010) 90B15, 90B20, 60K30
IntroductionThe study of heavy traffic in queueing systems began in the 1960s, with three pioneering papers by Kingman [26,27,28]. These papers, and the early work of Prohorov [35], Borovkov [5,6] and Iglehart [20], concerned a single resource. Since then there has been significant interest in networks of resources, with major advances by Harrison and Reiman [19], Reiman [37], Williams [43] and Bramson [7]. For discussions, further ref-erences and overviews of the very extensive literature on heavy traffic for networks, Williams [42], Bramson and Dai [8], Harrison [17, 18] and Whitt [41] are recommended.Research in this area is motivated in part by the need to understand and control the behaviour of communications, manufacturing and service networks, and thus to improve their design and performance. But researchers are also attracted by the elegance of some of the mathematical constructs: in particular, the multi-dimensional reflecting Brownian motions that often arise as limits.A question that arises in a wide variety of application areas concerns how flows through a network should be controlled, so that the network responds sensibly to varying conditions. Road traffic was an area of interest to early researchers [33], and more recently the question has been studied in work on modelling the Internet. In each of these cases the network studied is part of a larger system: for example, drivers generate demand and select their routes in ways that are responsive to the delays incurred or expected, which depend on the controls implemented in the road network. It is important to address such interactions between the network and the larger system, and in particular to understand the signals, such as delay, provided to the larger system.Work on Internet congestion control generally addresses the issue of fairness, since there exist situations where a given scheme might maximise network throughput, for example, while denying access to some users. In this area it has been possible to integrate ideas of fairness of a control scheme with overall system optimization: indeed fairness of the control scheme is often the means by which the right information and incentives are provided to the larger system [24,38].Might some of these ideas transfer to help our understanding of the control of road traffic? In this paper we present a...