Restricting ("metering") traffic flow on the approaches to an urban street network ("control area") can be considered an application of the concept of freeway ramp metering to surface street systems. In this application, local demand is reduced by metering traffic at the periphery of the control area during peak traffic demand periods. The purpose of this strategy is to maintain a level of traffic density within the control area to avoid congested flow conditions. It is postulated that if this objective is achieved, the performance of traffic will improve significantly within the control area and this improvement will more than offset the disbenefit associated with the possible delay of some traffic at the periphery. That is, the performance of the affected traffic, overall, will be improved. This paper presents the results of a simulation study that evaluated this hypothesis. On the basis of the results obtained in this study, it appears that the peripheral ("external") metering control strategies have the potential to improve the overall performance of traffic in a highly congested control area. The results indicate that it is virtually essential to apply a metering control along the periphery of a control area that is congested to the extent that the ensuing traffic demand cannot be serviced because of overflow queues causing extensive intersection spillback. It has also been shown that the optimal metering control policy to be enacted depends on the traffic condition before the implementation of such control (i.e., base condition) as well as the selected measure of effectiveness.
Abstract