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Variable Speed Limit Control to Mitigate Freeway Congestion Open Access


Other title
Variable speed limit
Model predictive control
Driver compliance level
Collision prediction model
Microscopic simulation
Type of item
Degree grantor
University of Alberta
Author or creator
Hadiuzzaman, Md
Supervisor and department
Qiu, Zhi-Jun (Department of Civil and Environmental Engineering)
Examining committee member and department
Chen, Yongsheng (City of Edmonton)
Kim, Amy (Department of Civil and Environmental Engineering)
El-Basyouny, Karim (Department of Civil and Environmental Engineering)
Zhong, Ming (Wuhan University of Technology)
Liu, Jinfeng (Department of Chemical and Materials Engineering)
Department of Civil and Environmental Engineering
Transportation Engineering
Date accepted
Graduation date
Doctor of Philosophy
Degree level
Over the past few decades, several active traffic control methods have been developed and implemented to mitigate freeway congestion. Among them, Variable Speed Limit (VSL) is considered the most efficient control method. In addition, the latest advances in Intelligent Transportation Systems (ITS) have made it feasible to implement predictive freeway control. The successful implementation of such control requires an accurate macroscopic traffic flow model that can predict all the important traffic dynamics. To avoid violation of the equilibrium traffic state assumption and to improve traffic state prediction accuracy in the VSL control situation, this research proposes a 2nd order model, DynaTAM-VSL, which drops parameterization of the METANET’s FD; instead, it includes speed limit-dependent parameters in the speed and density dynamics. The validation results with the 20-s loop detector data confirmed that, compared to the existing models, the proposed model better simulates traffic flow. With the validated model, this research investigates the impact of control parameters and demand levels on total travel time and throughput under the coordinated VSL control and determined a range of the demand / bottleneck capacity ratio, when VSL simultaneously improves both of the mobility parameters, which resolved the existing paradoxical results. This research also proposes an isolated VSL control strategy that aims at avoiding capacity drop at recurrent freeway bottlenecks. To evaluate the effectiveness of the control strategy, a base model of the 11-km test site: Whitemud Drive (WMD), Edmonton is calibrated within a microscopic traffic flow simulator to reproduce real-world traffic conditions, while the control strategy is implemented to evaluate its impact. The sensitivity analysis of the control strategy on safety constraints and VSL update frequencies demonstrates promising results to support practical implementation. Considering its flexible use in macroscopic simulation, a 1st order traffic flow model, CTM-VSL, is proposed. Unlike the 2nd order models, it is parsimonious: it only includes parameters that can be estimated using routinely available point detector data. However, the model is valid only for the condition of perfect compliance by drivers to VSL control, since it shares same properties of the CTM model. To update the storage capacity of an upstream segment of a VSL sign, a real-time queue estimation model is proposed. Despite the simple structure of the CTM-VSL model, the VSL control shows comparable results with the DynaTAM-VSL in terms of improving mobility parameters. Finally, this research distinguishes the relative contributions of driver compliance levels (CLs) and a predictive VSL control with different CLs to improve traffic flows. Several CL-to-VSL strategies are modeled with a fixed co-efficient of variance of speeds obtained from static speed limit on WMD. The CLs include speed distributions for aggressive, compliant, and defensive drivers. It is proven that the mobility benefits from the VSL control are not at the expense of increased collision probability and vice-versa.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
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