1,721,156 research outputs found
A feedback linearization approach for coordinated traffic flow management in highway systems
No full textIn this paper, a control solution to reduce congestion in highway traffic systems is presented. The aim is to produce a control strategy characterized by low computational cost, so that real-time implementation can be attained. The adopted model to describe traffic dynamics is the METANET model. A particular spatio-temporal derivative relationship, describing how control signals (ramp metering and variable speed limits) and disturbances effects propagate along the highway system, is highlighted in the paper. This relationship is the basis of a proposition providing the essential tool for relative degree calculation in generic highway systems. Utilizing this proposition, a feedback linearization-based control law is developed. The control design is completed by employing a linear MPC, which allows for complying with the physical constraints. The performance of the proposed method is evaluated by conducting comprehensive simulation studies, also considering a real-world traffic system. The computational costs are analyzed by comparing the developed methodology with a nonlinear MPC-based approach. Simulation evidence confirms that the proposed method can provide satisfactory solutions for coordinating RM and VSL in highway systems. Such solutions are compatible with real-time implementation
Large-scale traffic signal control and multimodal network design
No full textTraffic congestion constitutes one of the most frequent, yet challenging, problems to address in the urban space. Caused by the concentration of population, whose mobility needs surpass the serving capacity of urban networks, congestion cannot be resolved in the long term by creating more road space. Instead, network capacity can be increased by maximizing efficiency of traffic operations and road space use. Several methods can be employed for this purpose, including optimization of public transportation systems operations and intelligent, adaptive traffic signal control. This thesis contributes to the existing research in these directions, by developing and evaluating new modeling, optimization and adaptive signal control approaches, aiming at improving mobility in highly-congested, large-scale networks, while considering the dynamic characteristics of congestion propagation. The problem of optimal Dedicated Bus Lanes (DBL) location assignment in large networks with existing bus systems of fixed operational characteristics is addressed in chapter 2. A combinatorial optimization problem is formulated on the basis of an enhanced version of Store-and-Forward paradigm, a dynamic, queue-based macroscopic traffic model, able to properly capture the dynamics of backwards propagation of congestion due to queue spill-backs. Changes in mode choice equilibrium are considered in the evaluation of candidate solutions. An algorithmic scheme based on Local Search, problem-specific heuristics and Large Neighborhood Search (LNS) metaheuristic is developed to address the complex problem. Various destroy and repair operators for LNS are proposed, together with a learning process for assessing the importance of links in terms of receiving DBL, and a network decomposition strategy for accelerating the solution process for very large networks. A two-layer hierarchical traffic-responsive signal control framework is proposed in chapter 3, combining aggregated multi-region perimeter control (PC) with distributed Max Pressure (MP) control in isolated intersections. Partial deployment of MP in subsets of network nodes is performed and a methodology for identifying critical nodes for MP control based on node traffic characteristics is developed, in the scope of reducing MP implementation cost. Various node layouts of different network penetration rates are evaluated, both in independent MP application and as part of the combined framework. Simulation experiments are performed for a large-scale network of more than 1500 links and 900 intersections for two scenarios resulting in moderately and highly congested states, respectively. Results provide meaningful insights in terms of both independent and combined application of PC and efficient MP control. Under congested conditions, a properly selected subset of critical intersections with MP produces better performance than installing MP everywhere. Adding PC with MP creates even more significant improvements. Finally, detailed analysis of total remaining travel distance in multi-region networks is performed via microscopic simulation, in the scope of evaluating the benefits of utilizing the recently proposed M-Model, which is disconnected from the steady-state approximation of conventional PL model, in aggregated MFD-based network control applications. Results indicate significant potential improvement in terms of accuracy of prediction, especially in cases of highly-dynamic traffic evolution patterns.LUT
Real-time Estimation of Critical Values of the Macroscopic Fundamental Diagram for Maximum Network Throughput
No full textPerimeter flow control or gating has recently been found to be a practical and efficient control scheme in mitigating traffic congestion in urban road networks. This control scheme aims at stabilising the accumulation of vehicles (or a proxy of accumulation, e.g. average occupancy or density) of the macroscopic or network fundamental diagram near critical accumulation to achieve maximum network throughput. Nevertheless, the maximum throughput (capacity flow) in urban road networks may be observed over a range of accumulation-values. In this work, an extension of a previously proposed real-time feedback perimeter flow control strategy is proposed that allows the automatic monitoring of the critical accumulation to help maintain the accumulation near the optimal range of accumulation-values, while network’s throughput is maximised. To this end, we design a Kalman filter-based estimation algorithm that utilises real-time measurements of circu- lating flow and accumulation of vehicles to produce estimates of the currently prevailing critical accumulation. The developed strategy may be valuable whenever the network fundamental dia- gram is not well defined and the critical accumulation cannot accurately be specified or is subject change due to traffic-responsive signal control, traffic composition (e.g. cars versus buses), or non- recurrent day-to-day traffic patterns. We use real experimental data from an urban area with 70 sensors and show that the area exhibits a fundamental diagram with low scatter. We demonstrate that the fundamental diagram is reproduced under different days but its shape and critical occu- pancy depend on the applied semi-real-time signal control and the distribution of congestion in the network. Preliminary results from the application of the estimation algorithm to the experimental data indicate good estimation accuracy and performance, and rapid tracking behaviour
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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