Enhancing Ramp Metering Algorithms with the Use of Probability of Breakdown Models
Publication: Journal of Transportation Engineering
Volume 140, Issue 4
Abstract
Ramp-metering, which controls the onramp flow into the freeway, is successful in increasing freeway throughput and reducing overall travel-time. The maximum flow allowed from the onramp during ramp-metering is typically estimated so that the sum of flows from the onramp and mainline do not exceed a predetermined threshold (either the capacity of the downstream section or a threshold based on occupancy at capacity). Recent research has shown that this threshold is probabilistic and the transition from noncongested to congested conditions (i.e., breakdown) occurs stochastically. Also, research has shown that the contribution of the ramp and freeway demands on breakdown is different; 100 additional vehicles arriving from the ramp increase the probability of breakdown more than 100 additional vehicles from the freeway. This fluctuation has been studied through the development of breakdown probability models, which provide the probability of breakdown as a function of the combination of the mainline and ramp flows. The writers’ objective was to develop suitable site-specific probability of breakdown models and use them within existing ramp-metering algorithms to evaluate their ability to postpone the breakdown and reduce congestion at freeway facilities with recurring congestion. The writers first develop a process for obtaining breakdown-probability models for existing critical ramps (i.e., those where breakdown starts). Next, the writers propose specific enhancements to existing ramp-metering algorithms that incorporate probability-of-breakdown models. Proposed enhancements are presented for two algorithms, as follows: (1) the Minnesota stratified ramp-metering algorithm (SZM), and (2) the Ontario COMPASS algorithm. Simulation was used to replicate these algorithms and evaluate the proposed enhancements. The results of these experiments showed that the enhancements are effective in postponing congestion at the two sites by 17–35 min.
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Acknowledgments
The research reported in this paper was sponsored by the National Cooperative Highway Research Program (NCHRP) 3-87. The writers thank the NCHRP 3-87 panel for their input throughout this project. The writers also thank David Tsui and Phil Masters of the Ontario Ministry of Transportation (MTO), Simon Foo and Baher Abdulhai of the University of Toronto, and John Hourdos of the University of Minnesota for their assistance with data-gathering. The contents of this paper reflect the views of the writers, who are responsible for the facts and accuracy of the data presented in this paper.
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Published In
Journal of Transportation Engineering
Volume 140 • Issue 4 • April 2014
Copyright
© 2014 American Society of Civil Engineers.
History
Received: Feb 12, 2013
Accepted: Dec 3, 2013
Published online: Jan 24, 2014
Published in print: Apr 1, 2014
Discussion open until: Jun 24, 2014
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