1,720,970 research outputs found
Slot allocation on congested motorways: An alternative to congestion pricing
No full textWith respect to the prevailing congestion problems in the more urbanised regions of the European Union, transportation planners and policymakers are facing a dilemma. Supply-side measures, i.e. increasing the capacities, might shorten the congestion duration, especially if bottlenecks can be removed. However, the addition of extra lanes in motorway bottlenecks is generally very costly, not only in terms of investment costs, but also in terms of space consumption and environmental effects. The strategy to build to provide, that is, adjusting the capacities to the expected peak demand, is sub-optimal from an economic point of view. Economists have shown that users tend to respond to congestion in a way that is not optimal from a system point of view (see for instance Waiters, 1968). In many cases, there is a mismatch between the congestion costs experienced by an individual user and the congestion costs caused by the same user, and demand-side measures might help to stimulate 'social' behaviour. The problem is, however, that it is hard to convince the public that measures like congestion pricing are really effective in preventing congestion. The allegation that congestion pricing would only result in paid queuing instead of free queuing has successfully been used in a campaign against the introduction of road pricing in the Netherlands. The objections against congestion pricing, especially the doubts about the effectiveness of this solution, are less irrational than some advocates of the pricing mechanism try to let us believe. An example is the mismatch between the willingness- to-pay of a potential road user and the utility of the planned trip, which results primarily from income differences. A related problem is the redistribution of welfare. Many (potential) road users are 'direct losers'; that is, their direct transportation costs are higher, though this might be (partially) compensated by a form of tax refunds (Thomson, 1998). Even more convincing, however, is the argument that a knowledge gap prevents even the most flexible congestion-pricing scheme to be optimal. In the ease of simultaneous pricing, the congestion costs can be calculated exactly, but the users cannot base their choices on the exact costs (Thomson, 1998). Hence, the tolls will have to be based on demand estimations, and should not vary too much. This might result in under-utilisation of the road if the tolls are too high, or, more likely, congestion if the tolls are not high enough. Moreover, since users are not capable of perfectly performing the utility-maximisation task assumed in classical economic theory, road pricing will never be a perfect way to avoid congestion.Transport & PlanningCivil Engineering and Geoscience
Transport Infrastructure Slot Allocation
No full textIn this thesis, transport infrastructure slot allocation has been studied, focusing on selection slot allocation, i.e. on longer-term slot allocation decisions determining the traffic patterns served by infrastructure bottlenecks, rather than timetable-related slot allocation problems. The allocation of infrastructure capacity among carriers is a major issue in various transport infrastructure sectors, and therefore a theoretical framework on slot allocation would be desirable to support rational decision-making on slot allocation. The current state-of-the-art of slot allocation research does not provide such a theoretical framework, and therefore a theoretical framework to analyze slot allocation problems has been developed in this thesis. The first step in the development of a theoretical framework to analyze slot allocation problems has been the specification of a conceptual framework, which includes the definition of key concepts such as capacity. Capacity has been defined as being dependent on conditions such as composition of traffic and traffic context as well as assumptions about the desired balance between capacity and quality-of-service. The next step was to review the current application of slot allocation in the railway and aviation sectors, and the potential application of slot allocation in the road and navigation sectors. Slot allocation is currently applied in the railway and aviation sectors, and slot allocation may potentially be applied in other sectors. This thesis introduces the important distinction between selection and scheduling slot allocation. In both the railway and aviation sectors, the tradition has been to integrate selection and scheduling slot allocation. This thesis, however, considers selection slot allocation as a separate slot allocation level. Separating selection and scheduling slot allocation enables the application to each level of different rules with respect to slot validity, valuation of alternative slot requests, etc. The desired characteristics of selection slot allocation have been formulated in this thesis by analyzing the main desires of carriers and other interested parties such as shippers and authorities. It has been concluded that selection slots should be valid for a significantly longer period than scheduling slots, and a semi-static slot allocation procedure has been proposed. Furthermore, the acceptability principle has been introduced as a basis to specify desired slot size. However, the specification of standard basic slots by the allocation body (at different levels to attain differentiation of slot size) is desirable. A semi-static slot allocation procedure implies that selection slot allocation decisions may be based on an explicit evaluation of selection slot requests. The selection problem may be analyzed using congestion theory, resulting in a generic specification of traffic supply and demand. The next step is to specify traffic supply in more detail by specifying capacity constraints. Examining various types of primary traffic processes, traffic service processes, and traffic externalities, capacity constraints have been formulated, which may be applied to different types of bottlenecks. Three categories of capacity constraints have been distinguished, i.e. homogeneous capacity constraints, linear capacity constraints, and non-linear capacity constraints. The next step is the specification of objectives. The (primary) objective of slot allocation may usually be specified as a linear objective function. Depending on the type and number of capacity constraints, various instances of selection slot allocation decision problems may be formulated. The corresponding optimization problems may be solved using an exact solution algorithm, but for various reasons this thesis proposes a greedy approximation instead. Besides a standard greedy algorithm for selection problems with a single type of capacity constraint, an extended greedy algorithm has been developed to solve problems with two or more different types of capacity constraints. The latter algorithm has been tested for a hypothetical case study. Three main conclusions have been formulated in this thesis. The first main conclusion is that selection and scheduling should be considered as separate slot allocation levels having a hierarchical relationship. Selection slot allocation is of primary importance and scheduling slot allocation is only of secondary importance, because selection decisions determine which traffic is facilitated and which is not. The second main conclusion is that the validity of slots is a compromise between stability and flexibility. To ensure a sufficient level of stability, a validity of at least 5 years seems reasonable for selection slots. To ensure a reasonable level of flexibility, infinite validity of selection slots (historic rights) is not desirable, and at least every timetable season the opportunity should be offered to reserve selection slots. The final main conclusion is that the objec-tives and constraints of the selection problem can be modeled as linear functions, and the resulting binary linear programming problem can best be solved with the greedy efficiency algorithm presented in this thesis. This efficiency algorithm does not provide an exact solution of the binary linear programming problem, but its results are more robust and are easier to interpret than exact solution approaches.Civil Engineering and Geoscience
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
DNA translocation: The effect of excluded volume interaction
No full textHet sturen van DNA door een klein gaatje in een kunstmatig membraan biedt mogelijkheden voor het aflezen van een DNA streng. Tijdens dit proces beweegt de streng zich in een vloeistof, waardoor allerlei fysische effecten een rol gaan spelen. De krachten die op de streng werken zorgen ervoor dat de DNA streng zich van de ene kant van het membraan naar de andere kant van het membraan verplaatst. Tijdens die verplaatsing zal de keten met zichzelf en met moleculen uit de vloeistof botsen. Deze botsingen beinvloeden de tijd die de keten nodig heeft om van de ene kant van het membraan naar de andere kant van het membraan te komen. Die tijd wordt ook wel de translocatietijd genoemd. Hoe deze botsingen de translocatietijd beinvloeden, wordt in dit project onderzocht door de DNA streng te representeren door een ketting van kralen. Een computersimulatie toont aan dat de botsingen binnen de keten de translocatietijd laten afnemen. Tegelijkertijd zorgen de botsingen aan de kant van het membraan waar de ketting zich naartoe verplaatst voor een toename in translocatietijd.Bachelor TWEWIElectrical Engineering, Mathematics and Computer Scienc
Modeling Electromagnetic Fields in Strongly Inhomogeneous Media: An Application in MRI
No full textModeling electromagnetic fields in MRI involves two main challenges: the solution of the scattering problem resulting from Maxwell's equations has to be accurate and it has to be obtained within short computation time. In this thesis a method that meets both requirements is searched for. The method of moments with rooftop basis functions is used to discretize different formulations of the volume integral equation corresponding to Maxwell's equations. A simple two-layer cylinder test case is used to compare each solution with a derived analytical solution for scattering on a two-layer cylinder. Two types of errors are analyzed. The good performance of a staggered grid with respect to a non-staggered grid shows that the way of treating the mixed derivative terms is of great importance. The performance of a higher order approximation scheme on a non-staggered grid is close to the performance of a second order approximation scheme on a staggered grid. A contrast study shows that these two methods are particularly beneficial for high contrasts and on low resolution. The performance of the iterative solvers IDR(s) and GMRES is tested for each discretization method. IDR(4) shows excellent performance in reducing the computation time that is obtained with GMRES. Finally, human body simulations confirm the findings from the two-layer cylinder test case.Applied MathematicsNumerical AnalysisElectrical Engineering, Mathematics and Computer Scienc
Analysis of Quality of Service Mechanisms in IP and Road Networks
No full textNowadays, many infrastructures have to suffer from congestion problems due to a lack of capacity. The road infrastructure is a good example, but current telecommunications networks like the internet have to face the same problem. Quality of Service (QoS) is a concept in the internet, which has become more significant the last couple of years. QoS is the ability of a network element to have some level of assurance that its traffic and service requirements can be satisfied. Best-effort service is not able to satisfy such requirements. The road infrastructure seems to be a completely different infrastructure that can not be compared to an infrastructure like the internet. However, a closer study to both infrastructures reveals a lot of clear similarities. QoS mechanisms are already used or researched within both infrastructures. But the level of QoS seems to differ, especially in the field of guaranteed QoS, which is not yet attained in the road infrastructure. For this reason a proposal is made to introduce guaranteed QoS in the road infrastructure. This thesis starts with describing how QoS is, or could be defined in both infrastructures. The second part gives an overview of the several (QoS) mechanisms that are currently being applied or researched to cope with the mentioned congestion problems. The results are used to find correlations and to analyze which levels of QoS can be achieved with the current available concepts. Together with some knowledge of other infrastructures an important conclusion can be made; use of a reservation system seems to be inevitable to maintain Quality of Service, given capacity scarcity. The final chapter proposes an implementation of such a reservation system for the road infrastructure.Electrical Engineering, Mathematics and Computer ScienceTelecommunicatie- en Verkeersbegeleidingssysteme
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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