liu.seSearch for publications in DiVA
Change search
Refine search result
1 - 32 of 32
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • oxford
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Allström, Andreas
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Barceló, Jaume
    Department of Statistics and Operations Research, Universitat Politècnica de Catalunya.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Grumert, Ellen
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Gundlegård, David
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Traffic management for smart cities2017In: Designing, developing, and facilitating smart cities: urban design to IoT solutions. Part III / [ed] Vangelis Angelakis, Elias Tragos, Henrich C. Pöhls, Adam Kapovits and Alessandro Bassi, Switzerland: Springer, 2017, p. 211-240Chapter in book (Other academic)
    Abstract [en]

    Smart cities, participatory sensing as well as location data available in communication systems and social networks generates a vast amount of heterogeneous mobility data that can be used for traffic management. This chapter gives an overview of the different data sources and their characteristics and describes a framework for utilizing the various sources efficiently in the context of traffic management. Furthermore, different types of traffic models and algorithms are related to both the different data sources as well as some key functionalities of active traffic management, for example short-term prediction and control.

  • 2.
    Allström, Andreas
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Gundlegård, David
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ringdahl, Rasmus
    Linköping University, Department of Science and Technology, Communications and Transport Systems.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Bayen, Alexandre M.
    Department of Civil and Environmental Engineering, University of California.
    Patire, Anthony D.
    Department of Civil and Environmental Engineering, University of California.
    A hybrid approach for short-term traffic state and travel time prediction on highways2016In: TRB 95th annual meeting compendium of papers, 2016Conference paper (Refereed)
    Abstract [en]

    Traffic management and traffic information are essential in urban areas, and require a good knowledge about both the current and the future traffic state. Both parametric and non-parametric traffic state prediction techniques have previously been developed, with different advantages and shortcomings. While non-parametric prediction has shown good results for predicting the traffic state during recurrent traffic conditions, parametric traffic state prediction can be used during non-recurring traffic conditions such as incidents and events. Hybrid approaches, combining the two prediction paradigms have previously been proposed by using non-parametric methods for predicting boundary conditions used in a parametric method. In this paper we instead combine parametric and non-parametric traffic state prediction techniques through assimilation in an Ensemble Kalman filter. As non-parametric prediction method a neural network method is adopted, and the parametric prediction is carried out using a cell transmission model with velocity as state. The results show that our hybrid approach can improve travel time prediction of journeys planned to commence 15 to 30 minutes into the future, using a prediction horizon of up to 50 minutes ahead in time to allow the journey to be completed.

  • 3.
    Allström, Andreas
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Gundlegård, David
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ringdahl, Rasmus
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Bayen, Alexandre M.
    Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA.
    Patire, Anthony D.
    Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA.
    Hybrid Approach for Short-Term Traffic State and Travel Time Prediction on Highways2016In: Transportation Research Record, ISSN 0361-1981, E-ISSN 2169-4052, Vol. 2554, p. 60-68Article in journal (Refereed)
    Abstract [en]

    Traffic management and traffic information are essential in urban areas and require reliable knowledge about the current and future traffic state. Parametric and nonparametric traffic state prediction techniques have previously been developed with different advantages and shortcomings. While nonparametric prediction has shown good results for predicting the traffic state during recurrent traffic conditions, parametric traffic state prediction can be used during nonrecurring traffic conditions, such as incidents and events. Hybrid approaches have previously been proposed; these approaches combine the two prediction paradigms by using nonparametric methods for predicting boundary conditions used in a parametric method. In this paper, parametric and nonparametric traffic state prediction techniques are instead combined through assimilation in an ensemble Kalman filter. For nonparametric prediction, a neural network method is adopted; the parametric prediction is carried out with a cell transmission model with velocity as state. The results show that the hybrid approach can improve travel time prediction of journeys planned to commence 15 to 30 min into the future, with a prediction horizon of up to 50 min ahead in time to allow the journey to be completed

  • 4.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    A MILP approximation approach for finding optimal toll locations and levels in elastic demand traffic networks2010In: TRANSPORTATION AND URBAN SUSTAINABILITY: Proceedings of the 15th International Conference of Hong Kong Society for Transportation Studies (HKSTS) / [ed] Sumalee, A; Lam, WHK; Ho, HW; Siu, B, Hong Kong, China: Hong Kong Society for Transportation Studies , 2010, p. 107-114Conference paper (Refereed)
    Abstract [en]

    The toll design problem (TDP) is to find optimal toll locations and corresponding toll levels in a congestion pricing scheme. The TDP can be formulated as a non-convex mathematical program, in which the road users are assumed to be distributed according to a user-equilibrium with elastic demand. This program is hard to solve due to non-convexity and non-smoothness. In this paper, the TDP is approximated by a mixed integer linear program (MILP), in which the non-linear functions of the TDP are approximated by piecewise linear ones. The MILP can be solved to its global optimal solution by known methods, and its optimal solution will give a lower bound on the optimal solution to the TDP. By iteratively updating the MILP approximation, the error introduced by the approximation is reduced, and for a test network with nine nodes and 18 links, the global optimal solution is obtained.

  • 5.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Comparison of usage-based congestion pricing schemes2018In: Proceedings Transportation Research Board 97th Annual Meeting, Washington D.C., 2018Conference paper (Refereed)
    Abstract [en]

    This paper investigates how usage-based pricing can be applied in order to improve the efficiency of the transportation system. Distance-based, travel time-based and delayed-based pricing schemes are investigated, both from a one-link and network perspective. A static modeling framework is adopted, based on user-equilibrium, and a simplified emission model based on the HBEFA emission model is used. Optimal usage-based fees are calculated using a surrogate-based optimization framework, and determination of zone layout is done using k-means clustering. The results for a Stockholm region network show that already with network wide pricing a large proportion of the maximum improvement of social surplus achievable with first-best pricing, can be reached. Delay-based pricing, in which the users are charged based on the excess travel time in comparison to free flow travel times, achieves 99% of the maximum improvement, already with one single fee across the whole network. With clustering of links based on their first-best optimal fees, the benefits of both distance and travel time-based fees can significantly be improved.

  • 6.
    Ekström, Joakim
    Linköping University, Department of Science and Technology.
    Designing Urban Road Congestion Charging Systems: Models and Heuristic Solution Approaches2008Licentiate thesis, monograph (Other academic)
    Abstract [en]

    The question of how to design a congestion pricing scheme is difficult to answer and involves a number of complex decisions. This thesis is devoted to the quantitative parts of designing a congestion pricing scheme with link tolls in an urban car traffic network. The problem involves finding the number of tolled links, the link toll locations and their corresponding toll level. The road users are modeled in a static framework, with elastic travel demand.

    Assuming the toll locations to be fixed, we recognize a level setting problem as to find toll levels which maximize the social surplus. A heuristic procedure based on sensitivity analysis is developed to solve this optimization problem. In the numerical examples the heuristic is shown to converge towards the optimum for cases when all links are tollable, and when only some links are tollable.

    We formulate a combined toll location and level setting problem as to find both toll locations and toll levels which maximize the net social surplus, which is the social surplus minus the cost of collecting the tolls. The collection cost is assumed to be given for each possible toll location, and to be independent of toll level and traffic flow. We develop a new heuristic method which is based on repeated solutions of an approximation to the combined toll location and level setting problem. Also, a known heuristic method for locating a fixed number of toll facilities is extended, to find the optimal number of facilities to locate. Both heuristics are evaluated on two small networks, where our approximation procedure shows the best results.

    Our approximation procedure is also employed on the Sioux Falls network. The result is compared with different judgmental closed cordon structures, and the solution suggested by our method clearly improves the net social surplus more than any of the judgmental cordons.

  • 7.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Finding second-best toll locations and levels by relaxing the set of first-best feasible toll vectors2014In: European Journal of Transport and Infrastructure Research, ISSN 1567-7133, E-ISSN 1567-7141, Vol. 14, no 1, p. 7-29Article in journal (Refereed)
    Abstract [en]

    This paper provides a framework for optimizing toll locations and levels in congestion pricing schemes for large urban road networks, with the objective to maximize the social surplus. This optimization problem is referred to as the toll location and level setting problem (TLLP) and is both non-convex, non-smooth and involves binary decision variables, and is therefore considered as a hard problem to solve. In this paper a solution approach is provided which instead of directly solving the TLLP, makes use of the first-best toll level solution, in which no restrictions are imposed on toll locations or levels. A first-best pricing scheme can be obtained by solving a convex program, and it has previously been shown that for the used routes in the network, the first-best toll levels on a route level are unique. By formulating an optimization problem, which instead of maximizing the social surplus, tries to find the link toll levels which minimize the deviation from first-best route tolls, a mixed integer linear program is obtained, and if the toll locations are predetermined the resulting optimization problem is a linear program.

    The approach of minimizing the deviation from first--best route tolls is applied for two different network models, and results are provided to show the applicability of the approach, as well as to compare with other approaches. Also, it is shown that for the Stockholm network, virtually the first-best level of social surplus can be obtained with a significantly reduced number of located tolls.

  • 8.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Optimization Approaches for Design of Congestion Pricing Schemes2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In recent years, there has been a growing interest in congestion pricing as a tool for solving traffic congestion problems in urban areas. However, the transportation system is complex and to design a congestion pricing scheme, i.e. to decide where and how much to charge the road users, is not trivial. This thesis considers congestion pricing schemes based on road tolls, and the efficiency of a pricing scheme is evaluated by a social welfare measure. To assist in the process of designing congestion pricing schemes, the toll design problem (TDP) is formulated as an optimization problem with the objective function describing the change in social welfare. In the TDP, the road users are assumed to be distributed in the traffic network according to a Wardrop equilibrium. The TDP is a non-convex optimization problem, and its objective function is non-smooth. Thus, the TDP is considered as a hard optimization problem to solve.

    This thesis aims to develop methods capable of optimizing both toll locations and their corresponding toll levels for real world traffic networks; methods which can be used in a decision support framework when designing new congestion pricing schemes or tuning already implemented ones. Also, this thesis addresses the global optimality of the TDP. '

    In this thesis, a smoothening technique is applied which approximates the discrete toll location variables by continuous functions (Paper I). This allows for simultaneous optimization of both toll locations and their corresponding toll levels, using a sensitivity analysis based ascent algorithm. The smoothening technique is applied in a Stockholm case study (Paper II), which shows the potential of using optimization when designing congestion pricing schemes.

    Global optimality of the TDP is addressed by piecewise linear approximations of the non-linear functions in the TDP (Papers III and IV), resulting in a mixed integer linear program (MILP). The MILP can be solved to global optimality by branch and bound/cut methods which are implemented in commercially available software.

    List of papers
    1. Heuristic algorithms for a second-best congestion pricing problem
    Open this publication in new window or tab >>Heuristic algorithms for a second-best congestion pricing problem
    2009 (English)In: Netnomics, ISSN 1385-9587, E-ISSN 1573-7071, Vol. 10, no 1, p. 85-102Article in journal (Refereed) Published
    Abstract [en]

    Designing a congestion pricing scheme involves a number of complex decisions.Focusing on the quantitative parts of a congestion pricing system with link tolls, the problem involves findingthe number of toll links, the link toll locations and their corresponding toll level and schedule.In this paper, we develop and evaluate methods for finding the most efficient design for a congestion pricing scheme in a road network model with elastic demand. The design efficiency is measured by the net social surplus, which is computed as the difference between the social surplus and the collection costs (i.e. setup and operational costs) of the congestion pricing system. The problem of finding such a scheme is stated as a combinatorial bi-level optimization problem. At the upper level, we maximize the net social surplus and at the lower level we solve a user equilibrium problem with elastic demand, given the toll locations and toll levels,to simulate the user response. We modify a known heuristic procedure for finding the optimal locations and toll levels given a fixed number of tolls to locate, to find the optimal number of toll facilities as well. A new heuristic procedure, based on repeated solutions of a continuous approximation of the combinatorial problem is also presented. Numerical results for two small test networks are presented. Both methods perform satisfactorily on the two networks. Comparing the two methods, we find that the continuous approximation procedure is the one which shows the best results.

    Keywords
    transport modeling, congestion pricing, network design, bi-level optimization, toll locations, traffic assignment, user equilibrium, collection cost
    National Category
    Other Engineering and Technologies not elsewhere specified
    Identifiers
    urn:nbn:se:liu:diva-18641 (URN)10.1007/s11066-008-9019-9 (DOI)
    Available from: 2009-06-03 Created: 2009-06-03 Last updated: 2017-12-13Bibliographically approved
    2. Optimal Toll Locations and Levels in Congestion Pricing Schemes: a Case Study of Stockholm
    Open this publication in new window or tab >>Optimal Toll Locations and Levels in Congestion Pricing Schemes: a Case Study of Stockholm
    2014 (English)In: Transportation planning and technology (Print), ISSN 0308-1060, E-ISSN 1029-0354, Vol. 37, no 4, p. 333-353Article in journal (Refereed) Published
    Abstract [en]

    As congestion pricing has moved from theoretical ideas in the literature to real world implementations, the need for decision support when designing the pricing schemes has become evident. This paper deals with the problem of finding optimal toll levels and locations in a road traffic network, and presents a case study of Stockholm. The optimization problem of finding optimal toll levels, given a predetermined cordon, and the problem of finding both optimal toll locations and levels are presented, and previously developed heuristics are used for solving these problems. For the Stockholm case study, the possible welfare gains of optimizing the toll levels in the current cordon, and optimizing both the toll locations and their corresponding toll levels are evaluated. It is shownthat by tuning the toll levels in the current congestion pricing cordon used in Stockholm, the welfare gain can be significantly increased, and furthermore improved by allowing a toll on the bypass highway “Essingeleden”. It is also shown that by optimizing both the toll locations and levels, a congestion pricing scheme with welfare gain close to what can be achieved by marginal social cost pricing, can be designed with tolls being located on only a forth of the tollable links.

    Place, publisher, year, edition, pages
    Routledge, 2014
    Keywords
    congestion pricing; road tolls; bilevel optimisation; user equilibrium; network design; Stockholm
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-76640 (URN)10.1080/03081060.2014.897129 (DOI)000334158100002 ()
    Note

    On the day of the defence date of the Thesis the status of this article was Manuscript.

    Available from: 2012-04-13 Created: 2012-04-13 Last updated: 2017-12-07Bibliographically approved
    3. Optimizing Toll Locations and Levels Using a Mixed Integer Linear Approximation Approach
    Open this publication in new window or tab >>Optimizing Toll Locations and Levels Using a Mixed Integer Linear Approximation Approach
    2012 (English)In: Transportation Research Part B: Methodological, ISSN 0191-2615, E-ISSN 1879-2367, Vol. 46, no 7, p. 834-854Article in journal (Refereed) Published
    Abstract [en]

    This paper addresses the toll design problem of finding the toll locations and levels in a congestion pricing scheme, which minimize the total travel time and the toll-point cost (set-up and operational costs of the toll collecting facilities). Road users in the network are assumed to be distributed according to the principle of user equilibrium, with the demand assumed to be fixed and given a priori. The toll design problem is commonly formulated as a nonlinear program, which in general is non-convex and non-smooth, and thus difficult to solve for a global optimum. In this paper, the toll design problem is approximated by a mixed integer linear program (MILP), which can be solved to its globally optimal solution. The MILP also gives a lower bound estimation of the original non-linear problem, and the accuracy of the approximation is improved by iteratively updating the MILP. To demonstrate the approach, we apply the algorithm to two networks: a smaller network with 18 links and 4 OD-pairs to illustrate the properties of the approach, and the Sioux Falls network with 87 links and 30 OD-pairs to demonstrate the applicability of the approach.

    Place, publisher, year, edition, pages
    Elsevier, 2012
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-76641 (URN)10.1016/j.trb.2012.02.006 (DOI)000305435600004 ()
    Available from: 2012-04-13 Created: 2012-04-13 Last updated: 2017-12-07Bibliographically approved
    4. Solving a Mixed Integer Linear Program Approximation of the Toll Design Problem Using Constraint Generation within a Branch and Cut Algorithm
    Open this publication in new window or tab >>Solving a Mixed Integer Linear Program Approximation of the Toll Design Problem Using Constraint Generation within a Branch and Cut Algorithm
    2014 (English)In: Transportmetrica A: Transport Science, ISSN 2324-9935, Vol. 10, no 9, p. 791-819Article in journal (Refereed) Published
    Abstract [en]

    This paper addresses the global optimality of the toll design problem (TDP) by a mixed integer linear program (MILP) approximation. In the TDP, the objective is to maximize the social surplus by adjusting toll locations and levels in a road traffic network. The resulting optimization problem can be formulated as a mathematical program with equilibrium constraints (MPEC).

    A MILP is obtained by piecewise linear approximation of the non-linear functions in the TDP, and we present a domain reduction scheme to reduce the error introduced by these approximations. Previous approaches for solving the MILP approximation have been relying on a large number of MILPs to be solved iteratively within a cutting constraint algorithm (CCA). This paper instead focuses on the development of a solution algorithm for solving the MILP approximation in which the CCA is integrated within a branch and cut algorithm, which only requires one MILP to be solved.

    Place, publisher, year, edition, pages
    Taylor & Francis Group, 2014
    Keywords
    global optimisation; bilevel optimisation; toll design; road pricing
    National Category
    Civil Engineering
    Identifiers
    urn:nbn:se:liu:diva-76642 (URN)10.1080/23249935.2013.813988 (DOI)000340257500002 ()
    Note

    The original titel in manuscript form was Solving a MILP Approximation of the Toll Design Problem Using Constraint Generation within a Branch and Cut Algorithm.

    Available from: 2012-04-13 Created: 2012-04-13 Last updated: 2014-10-23
  • 9.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Optimizing toll locations and levels in a road congestion pricing schemes by minimizing the deviation from marginal social cost pricing tolls2012In: Transportation & Logistics Management: Proceedings of the 17th International Conference of Hong Kong Society for Transportation Studies (HKSTS) : 15-17 December 2012, Hong Kong / [ed] Ho-Yin Mak and Hong K. Lo, Hong Kong: Hong Kong Society for Transportation Studies (HKSTS), 2012Conference paper (Other academic)
    Abstract [en]

    To find second-best optimal toll locations and toll levels in a road traffic network, which maximize the social surplus, can be formulated as a bi-level optimization program. The bi-level program is both non-convex and non-smooth and therefore considered as a demanding optimization problem to solve. First-best optimal tolls can, however, easily be computed by solving a convex program, and this paper examines the possibility to make use of these first-best tolls when searching for second-best solutions. By formulating an optimization problem, which instead of maximizing the social surplus, search for the toll locations and toll levels which minimize the deviation from first-best tolls on a route-level, a mixed integer linear program is formulated. The approach of minimizing the deviation from first-best route tolls is applied to a Stockholm network model, and results are provided to evaluate the applicability of the approach.

  • 10.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Optimizing toll locations and levels in a road congestion pricing schemes by minimizing the deviation from marginal social cost pricing tolls2012Conference paper (Other academic)
    Abstract [en]

    The problem of designing a congestion pricing system, assuming that the road users are distributed according to a static user equilibrium, is commonly formulated as a bi-level program, with the toll locations and toll levels being the decision variables. The upper level objective is to maximize the social surplus, and on the lower level a convex program is solved to obtain the road users response to the toll level solution. This optimization problem is both non-convex and non-smooth and therefore difficult to solve for a global optimum.

     

    For one special case, when all links are tollable and when there are no restrictions on the toll levels, the bi-level program can be formulated as a convex program which can easily be solved, with one optimal solution, among possibly several ones, equal to marginal cost pricing tolls (MSCP). With MSCP tolls, the road users are charged a toll on each congested road segment equal to the marginal increase in total travel cost incurred by one additional road user on the road segment. Any solution which maximizes the social surplus functions is referred to as a first-best toll level solution (as opposed to second-best when there are restrictions on toll locations and/or toll levels) with system optimal link flows and demands. While there may exist several first-best toll level solutions, they all satisfy the principle of MSCP on a route level, if the demand in the traffic network is elastic. This result has previously in the literature been used to devise a set of feasible toll vectors which all result in system optimal flows and demands.

     

    In this paper the set of feasible first-best toll vectors is relaxed to allow second-best solutions. The relaxation is performed on a route level, and the deviation from first-best route tolls is penalized. The objective then becomes to minimize the penalty, weighted by some factor to reflect each routes importance. The optimization problem either takes the form of a linear program (LP), if the toll locations are fixed, or a mixed integer linear program (MILP), if the toll locations are variable. For solutions with optimal objective function value close to zero, the toll level solution will be close to the global maximizer of the social surplus function. As the number of tollable links is reduced, the true equilibrium demands and link flows may differ from the system optimal ones. Thus, the performance of the approach can be assumed to be dependent of the number of tollable links. Numerical results show that the approach can give valuable information on close to optimal toll locations and toll levels, even when a small subset of the links are tollable.

     

     

     

  • 11.
    Ekström, Joakim
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    Engelson, Leonid
    Centre for Traffic Research KTH.
    Rydergren, Clas
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    A heuristic method for finding congestion pricing schemes in transportation networks with modal choice2008In: International conference of Hong Kong society for transportation studies,2008 / [ed] Lo, HP; Leung, SCH; Tam, SML, Hong Kong, China: Hong Kong Society of Transportation Studies Ltd. , 2008, p. 773-782Conference paper (Refereed)
    Abstract [en]

     In this paper we extend a previously developed heuristic procedure, with a modal choice model, to solve the congestion pricing problem of simultaneously finding the optimal number of toll facilities, their corresponding location and toll levels. When considering a congestion pricing scheme the cost of collecting the tolls can not be disregarded. The objective is where-fore to maximize the net social surplus, which is the social surplus minus the cost of collect-ing the tolls. The heuristic method is an iterative solution procedure, in which the integer part of the objec-tive function is approximated by a continuous function. A version of the Sioux Falls network (76 links) is used to demonstrate the solution procedure. The solution is a congestion pricing scheme which divide the network into four zones, by locating tolls on 27 links. This solution yields a social surplus which is only 13.5% lower than the marginal social cost pricing solu-tion. 

  • 12.
    Ekström, Joakim
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    Engelson, Leonid
    Centre for Traffic Research KTH.
    Rydergren, Clas
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    Decision support for finding locations and toll levels within a congestion pricing scheme2007In: Kuhmo Nectar Conference,2007, 2007Conference paper (Other academic)
  • 13.
    Ekström, Joakim
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    Engelson, Leonid
    Centre for Traffic Research KTH.
    Rydergren, Clas
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    Decision support for finding locations and toll levels within a congestion pricing scheme2008In: Transportation Research Board Annual Meeting,2008, Washington D.C.: TRB , 2008Conference paper (Refereed)
    Abstract [en]

    Designing a congestion pricing scheme involves a number of complex decisions. Focusing on the quantitative parts of a congestion pricing system with link tolls, the problem involves finding the number of tolled links, the link toll locations and their corresponding toll level and schedule. In this paper, we develop and evaluate methods for finding a most efficient design of a congestion pricing scheme in a road network with elastic demand. The design efficiency is measured by the net social surplus, which is computed as the difference between the social surplus and the collection costs (i.e. setup and operation cost) of the congestion pricing system. The problem of finding such a scheme is stated as a combinatorial bi-level optimization problem. On the upper level we maximize the net social surplus and on the lower level we solve a user equilibrium problem with elastic demand, given the toll locations and toll levels, to simulate the user response. We modify a known heuristic procedure for finding the optimal locations and toll levels given a fixed number of tolls to locate, to find the optimal number of tolls to locate as well. A new heuristic procedure is also presented which is based on repeated solutions of a continuous approximation of the combinatorial problem. Numerical results for two small scale test networks are presented. Both methods perform satisfactory on the two networks. Comparing the two methods, the iterative approximation procedure is the one which shows the best results. The results are compared to solutions obtained by an exhaustive search.

  • 14.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Engelson, Leonid
    Centre for Traffic Research, Royal Institute of Technology.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Heuristic algorithms for a second-best congestion pricing problem2009In: Netnomics, ISSN 1385-9587, E-ISSN 1573-7071, Vol. 10, no 1, p. 85-102Article in journal (Refereed)
    Abstract [en]

    Designing a congestion pricing scheme involves a number of complex decisions.Focusing on the quantitative parts of a congestion pricing system with link tolls, the problem involves findingthe number of toll links, the link toll locations and their corresponding toll level and schedule.In this paper, we develop and evaluate methods for finding the most efficient design for a congestion pricing scheme in a road network model with elastic demand. The design efficiency is measured by the net social surplus, which is computed as the difference between the social surplus and the collection costs (i.e. setup and operational costs) of the congestion pricing system. The problem of finding such a scheme is stated as a combinatorial bi-level optimization problem. At the upper level, we maximize the net social surplus and at the lower level we solve a user equilibrium problem with elastic demand, given the toll locations and toll levels,to simulate the user response. We modify a known heuristic procedure for finding the optimal locations and toll levels given a fixed number of tolls to locate, to find the optimal number of toll facilities as well. A new heuristic procedure, based on repeated solutions of a continuous approximation of the combinatorial problem is also presented. Numerical results for two small test networks are presented. Both methods perform satisfactorily on the two networks. Comparing the two methods, we find that the continuous approximation procedure is the one which shows the best results.

  • 15.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Engelson, Leonid
    Centre for Traffic Research, Royal Institute of Technology, Stockholm, Sweden.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Optimal Toll Locations and Levels in Congestion Pricing Schemes: a Case Study of Stockholm2014In: Transportation planning and technology (Print), ISSN 0308-1060, E-ISSN 1029-0354, Vol. 37, no 4, p. 333-353Article in journal (Refereed)
    Abstract [en]

    As congestion pricing has moved from theoretical ideas in the literature to real world implementations, the need for decision support when designing the pricing schemes has become evident. This paper deals with the problem of finding optimal toll levels and locations in a road traffic network, and presents a case study of Stockholm. The optimization problem of finding optimal toll levels, given a predetermined cordon, and the problem of finding both optimal toll locations and levels are presented, and previously developed heuristics are used for solving these problems. For the Stockholm case study, the possible welfare gains of optimizing the toll levels in the current cordon, and optimizing both the toll locations and their corresponding toll levels are evaluated. It is shownthat by tuning the toll levels in the current congestion pricing cordon used in Stockholm, the welfare gain can be significantly increased, and furthermore improved by allowing a toll on the bypass highway “Essingeleden”. It is also shown that by optimizing both the toll locations and levels, a congestion pricing scheme with welfare gain close to what can be achieved by marginal social cost pricing, can be designed with tolls being located on only a forth of the tollable links.

  • 16.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems.
    Engelson, Leonid
    Royal Institute of Technology.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems.
    TOWARDS OPTIMAL LOCATIONS AND TOLLLEVELS IN CONGESTION PRICING SCHEMES2009In: Proceedings of the 16th ITS World Congress, 2009Conference paper (Refereed)
    Abstract [en]

    This paper addresses the problem of designing a road congestion pricing scheme withlink tolls. The problem involves decisions on where to locate the toll collecting facilitiesand what tolls to charge the road users. We formulate this problem as a bi-levelprogram, with the objective to maximize the net social surplus, which include the costof setting up and operate the toll collection system. A previously developed heuristicmethod is applied to find close to optimal toll locations and charges for a traffic networkrepresenting the Stockholm region. The result is compared with the current congestionpricing scheme in Stockholm.

  • 17.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Kristoffersson, Ida
    Trafikanalys, Sweco TransportSystem AB, Stockholm.
    Quttineh, Nils-Hassan
    Linköping University, Department of Mathematics, Optimization . Linköping University, Faculty of Science & Engineering.
    Surrogatbaserad optimering av avgiftsnivåer i trängselavgiftssystem2015Report (Other academic)
    Abstract [sv]

    Trängselskatt finns idag i både Stockholm och Göteborg, och det är troligt att utformningen av dessa trängselskattesystem kommer att justeras framöver med avseende på avgiftsnivå, placering och tidpunkt. För Stockholm finns beslut om ändring från januari 2016 och i Göteborg ändrades avgiftsnivåerna i januari 2015. I detta projekt utvecklas metoder som ska kunna ge stöd vid justering av avigiftsnivåer, så att en så stor samhällsekonomisk nytta som är möjligt uppnås med trängselskattesystemet.

    För storstadsområden, där det under rusningstrafik är trängsel i delar av nätverket, är trängselskatt främst intressant att analysera med dynamiska transportmodeller. Tidigare utveckling av metoder för optimal avgiftssättning har dock främst fokuserat på statiska modeller, exempelvis Emme, som har kända problem med att korrekt uppskatta förändring i restider när det är trängsel i delar av trafiknätverket. I detta projekt har vi därför tillämpat surrogat-baserad optimering, som är en metodansats som ställer få krav på vilken transportmodell som används. Den dynamiska transportmodellen Regent/VisumDUE finns sedan tidigare implementerad för Stockholmsregionen, och har därför även använts i detta projekt. VisumDUE är en makroskopisk nätutläggningsmodell med dynamiskt ruttval, och Regent är en efterfrågemodell som innehåller resgenerering, färdmedelsval och destinationsval för arbetsresor[1].

    Surrogat-baserad optimering erbjuder ett ramverk för optimering av problem med beräkningsmässigt kostsamma målfunktioner. Genom att approximera en funktionsyta till samplade punkter från den kostsamma målfunktionen, kan optimeringen istället göras över den approximerade funktionsytan. För Regent/VisumDUE tar utvärderingen av ett givet trängselskattescenario ca tio timmar, och det är denna beräkningstid som gör målfunktionen kostsam. Givet ett antal samplade punkter, görs ytterligare sampling utifrån en given strategi för att förbättra approximationen, så kallad iterativ sampling. Inom ramverket finns dock en mängd möjligheter för hur de olika komponenterna designas. Därför är det svårt att utvärdera surrogat-baserad optimering med endast Regent/VisumDUE. En statisk transportmodell har därför använts för att utvärdera ett antal kombinationer av samplingsstrategi och funktionsyta. Den mest lovande kombinationen har sedan även utvärderats med Regent/VisumDUE. För att vara praktiskt tillämpbart i framtiden har fokus i projektet varit att utvärdera hur metodansatsen fungerar när antalet möjliga tulluppsättningar är kraftigt begränsat (20-40 stycken).

    Det scenario som har använts som grund i projektet är trängselskatt i Stockholm på nuvarande tullring, på Essingeleden samt på innerstadsbroarna. Skatten är differentierad med avseende på riktning, vilket ger sex olika skattenivåer att optimera. Optimeringen har gjorts för trängselskattenivå under maxtimmen. I det dynamiska fallet har trängselskattens nivå utanför maxtimme funnits med som indata, men samma tidsprofil som på nuvarande tullring har antagits i alla scenarier (avgiftstrappa 50%, 75%, 100%, 75%, 50%). Utvärderingen med den statiska transportmodellen visar att lösningar nära globalt optimum kan uppnås med endast 40 utvärderade trängselskattenivåer, och en tydlig förbättring av den samhällsekonomiska nyttan uppnås redan vid 20 utvärderade trängselskattenivåer.

    Även med ett kraftigt begränsat antal utvärderingar av den kostsamma målfunktionen i Regent/VisumDUE, har vi visat att det är möjligt att använda metodansatsen. En tydlig förbättring av den samhällsekonomiska nyttan uppnås med endast 22 utvärderade trängselskattenivåer. Ytterligare experiment skulle dock behövas för att undersöka hur stor denna förbättring är i förhållande till vad som skulle kunna uppnås.

  • 18.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Kristoffersson, Ida
    Centre for Transport Studies, KTH Royal Institute of Technology, Stockholm, Sweden.
    Quttineh, Nils-Hassan
    Linköping University, Department of Mathematics, Optimization . Linköping University, Faculty of Science & Engineering.
    Surrogate-based optimization of cordon toll levels in congested traffic networks2016In: Journal of Advanced Transportation, ISSN 0197-6729, E-ISSN 2042-3195, Vol. 50, no 6, p. 1008-1033Article in journal (Refereed)
    Abstract [en]

    The benefits, in terms of social surplus, from introducing congestion pricing schemes in urban networks are depending on the design of the pricing scheme. The major part of the literature on optimal design of congestion pricing schemes is based on static traffic assignment, which is known for its deficiency in correctly predict travels in networks with severe congestion. Dynamic traffic assignment can better predict travel times in a road network, but are more computational expensive. Thus, previously developed methods for the static case cannot be applied straightforward. Surrogate-based optimization is commonly used for optimization problems with expensive-to-evaluate objective functions. In this paper we evaluate the performance of a surrogate-based optimization method, when the number of pricing schemes which we can afford to evaluate (due to the computational time) is limited to between 20 and 40. A static traffic assignment model of Stockholm is used for evaluating a large number of different configurations of the surrogatebased optimization method. Final evaluation is done with the dynamic traffic assignment tool VisumDUE, coupled with the demand model Regent, for a Stockholm network including 1 240 demand zones and 17 000 links. Our results show that the surrogate-based optimization method can indeed be used for designing a congestion pricing scheme which return a high social surplus.

  • 19.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Quttineh, Nils-Hassan
    Linköping University, Department of Mathematics, Optimization . Linköping University, The Institute of Technology.
    Surrogate-based optimisation of toll levels in congestion pricing schemes2014In: Transportation Infrastructure: Proceedings of the 19th International Conference of Hong Kong Society for Transportation Studies (HKSTS) / [ed] Z. Leng and Y. H. Wang, Hong Kong: Hong Kong Society of Transportation Studies Limited , 2014, p. 209-216Conference paper (Refereed)
    Abstract [en]

    There has recently been a growing interest in analysing road pricing schemes in urban areas using dynamic traffic assignment (DTA) tools. Finding optimal toll levels in cordon based road pricing schemes has so far mainly been studied using either derivative-free heuristics or ascent methods. For future use of DTA tools such methods are not suitable and in this paper we investigate how a surrogate modelling framework can be used instead. We focus on cases when the number of costly objective function evaluations is limited to between 20 and 40. In order to allow a large number of different configurations of the surrogate modelling framework to be evaluated, a static user equilibrium model is used for simulating the road users’ response to a given pricing scheme. The results show that for a realistic scenario, valuable information on close to optimal toll levels can be achieved with only 20 costly function evaluations.

  • 20.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Quttineh, Nils-Hassan
    Linköping University, Department of Mathematics, Optimization . Linköping University, The Institute of Technology.
    Kristoffersson, Ida
    Sweco TransportSystem.
    Simulation based optimisation of toll levels in urban road traffic networks2014Conference paper (Other academic)
    Abstract [en]

    There has recently been a growing interest in analysing road pricing schemes in urban areas using dynamic traffic assignment (DTA) tools. The motivation behind this development is the problem for static transportation models to accurately predict travel time savings, from introducing road pricing, in networks with severe congestion. Finding optimal toll levels and locations in urban road traffic networks has so far mainly been studied using either derivative-free heuristics (e.g. genetic algorithms and simulated annealing) or ascent methods. Both approaches rely on fast computations of the road users response (traffic flows, travel times and demands), given the road pricing scheme, and for the case of ascent methods, the methods also rely on fast computations (or rather approximation) of derivatives. Using DTA tools for evaluating the road users’ response to a pricing scheme is, however, very computationally expensive. Previously developed methods are therefore not suitable to use together with DTA.

    Surrogate models, e.g. in terms of response surfaces, are commonly used for optimisation problems with expensive-to-evaluate objective functions. The surrogate model is used for approximating the expensive-to-evaluate objective function, and the optimisation is then done on the surrogate model instead. The performances of optimisation methods based on surrogate models are, however, dependent on experimental design, infill strategy and choice of surrogate model itself. The experimental design will give the initial set of toll levels, for which the DTA needs to be evaluated, the infill strategy determined additional toll levels to be evaluated by the DTA, and the choice of surrogate model will give the functional form to be fitted to the sampled toll levels.

    We apply a surrogate model framework for optimising toll levels in a multiple cordon pricing scheme. In the first stage we evaluate the experimental design, infill strategy and choice of surrogate model, using a static macroscopic traffic model.  This allows a large number of experiments to be carried out, which would not be possible with a DTA tool. It also allows us to compare the performance of the surrogate modelling approach with other global optimisation methods. In the second stage, the insight which has been gained from the experiments with the static model is used when applying the surrogate modelling approach to a DTA model of Stockholm.

    Computational results are presented for a Stockholm network with three cordons, each with differentiated toll level in both directions, resulting in a total of six toll level variables. Surrogate models in the form of Radial Basis Functions and Kriging models are evaluated with a static model of Stockholm, for different initial experimental designs, infill strategies and choice of surrogate models. In comparison with previously developed derivative based methods for static models, our results show that the surrogate based optimisation approach performs better, since it allows for metaheuristic methods to search for global optimal solutions efficiently.

  • 21.
    Ekström, Joakim
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    Rydergren, Clas
    Linköping University, The Institute of Technology. Linköping University, Department of Science and Technology.
    Decision support for finding optimal locations and toll levels within a congestion pricing scheme2007In: Second Nordic Optimization Symposium,2007, 2007Conference paper (Other academic)
  • 22.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Why modeling of heavy goods vehicles matters when designing congestion pricing schemes2013Conference paper (Other academic)
    Abstract [en]

    Differentiated tolls for cars and heavy goods vehicles (HGVs) may improve the efficiency of congestion pricing schemes. Not only do private trips by car and commercial activities by HGV differ in how the time is valued by the user/operator, in general, one additional truck will also contribute to the congestion more than one additional car. In this paper we show how different modeling approaches for HGV route choice and demand may affect the evaluation of road pricing schemes. Also, we provide a small example to illustrate potential problems, and present results for a larger model of the German city Stuttgart.

  • 23.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Sumalee, Agachai
    The Hong Kong Polytechnic University, Department of Civil and Structural Engineering.
    Finding optimal toll locations and levels in elasticdemand networks: -A MILP approximation approach2010In: Proceedings of the Tristan VII Conference, 2010Conference paper (Refereed)
  • 24.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Rydergren, Clas
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Sumalee, Agachai
    Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong, China.
    Solving a Mixed Integer Linear Program Approximation of the Toll Design Problem Using Constraint Generation within a Branch and Cut Algorithm2014In: Transportmetrica A: Transport Science, ISSN 2324-9935, Vol. 10, no 9, p. 791-819Article in journal (Refereed)
    Abstract [en]

    This paper addresses the global optimality of the toll design problem (TDP) by a mixed integer linear program (MILP) approximation. In the TDP, the objective is to maximize the social surplus by adjusting toll locations and levels in a road traffic network. The resulting optimization problem can be formulated as a mathematical program with equilibrium constraints (MPEC).

    A MILP is obtained by piecewise linear approximation of the non-linear functions in the TDP, and we present a domain reduction scheme to reduce the error introduced by these approximations. Previous approaches for solving the MILP approximation have been relying on a large number of MILPs to be solved iteratively within a cutting constraint algorithm (CCA). This paper instead focuses on the development of a solution algorithm for solving the MILP approximation in which the CCA is integrated within a branch and cut algorithm, which only requires one MILP to be solved.

  • 25.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems.
    Sumalee, Agachai
    Department of Civil and Structural Engineering, Hong Kong Polytechnic University.
    Optimizing toll levels using linear approximation approach2009In: 4th Kuhmo Nectar Conference, 2009Conference paper (Refereed)
    Abstract [en]

    This paper proposes a heuristic solution algorithm for solving the non-convex toll level problem for fixed demand networks in which the road users are distributed according to a user equilibrium. In the toll level problem we search for continuous toll levels, given a fixed set of tollable links, to minimize the total travel time in the traffic network. The toll level problem is converted by a linearization scheme to approximate the objective function and constraints in the original problem. This approximation gives a mixed integer linear program (MILP) which has the property of global optimum, and gives a lower bound estimation of the original non-linear problem. The user equilibrium condition is represented by the variational inequality (VI) constraints, and the MILP approximation is solved by applying a cutting constraint algorithm (to deal with the VI-constraints) together with a commercial MILP-solver. Numerical results are presented for a small network, and the results are encouraging.

  • 26.
    Ekström, Joakim
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Sumalee, Agachai
    Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong, China.
    Lo, Hong K.
    Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong.
    Optimizing Toll Locations and Levels Using a Mixed Integer Linear Approximation Approach2012In: Transportation Research Part B: Methodological, ISSN 0191-2615, E-ISSN 1879-2367, Vol. 46, no 7, p. 834-854Article in journal (Refereed)
    Abstract [en]

    This paper addresses the toll design problem of finding the toll locations and levels in a congestion pricing scheme, which minimize the total travel time and the toll-point cost (set-up and operational costs of the toll collecting facilities). Road users in the network are assumed to be distributed according to the principle of user equilibrium, with the demand assumed to be fixed and given a priori. The toll design problem is commonly formulated as a nonlinear program, which in general is non-convex and non-smooth, and thus difficult to solve for a global optimum. In this paper, the toll design problem is approximated by a mixed integer linear program (MILP), which can be solved to its globally optimal solution. The MILP also gives a lower bound estimation of the original non-linear problem, and the accuracy of the approximation is improved by iteratively updating the MILP. To demonstrate the approach, we apply the algorithm to two networks: a smaller network with 18 links and 4 OD-pairs to illustrate the properties of the approach, and the Sioux Falls network with 87 links and 30 OD-pairs to demonstrate the applicability of the approach.

  • 27.
    Hadera, Hubert
    et al.
    Technical University of Dortmund, Germany.
    Wide, Per
    Linköping University.
    Harjunkoski, Iiro
    ABB Corporate Research, Germany.
    Mäntysaari, Juha
    ABB Oy Industry Solutions, Finland.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Sand, Guido
    ABB Corporate Research, Germany.
    Engell, Sebastian
    Technical University of Dortmund, Germany.
    A Mean Value Cross Decomposition Strategy for Demand-side Management of a Pulping Process2015In: 12th International Symposium on Process Systems Engineering and 25th European Symposium on Computer Aided Process Engineering / [ed] Krist V. Gernaey, Jakob K. Huusom and Gani Rafiqul, Elsevier , 2015, Vol. 37, p. 1931-1936Chapter in book (Refereed)
    Abstract [en]

    Energy is becoming a critical resource for process industries as introduction of new policies drive changes in the energy supply systems. Energy availability and pricing is much more volatile. In this study, we propose a Mean Value Cross Decomposition approach to functionally separate production scheduling from energy-cost optimization. Such a decomposition makes it possible to exploit existing optimization solutions avoiding a need to create a new monolithic model. The proposed framework is applied to a continuous process of thermo-mechanical pulping using a discrete-time Resource-Task Network model. Example case study scenarios show that the approach gives optimal system-wide solutions while keeping the models separated.

  • 28.
    Häll, Carl Henrik
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ceder, Avishai (Avi)
    Transportation Research Center, Department of Civil and Environmental Engineering, 17 20 Symonds Street, The University of Auckland, New Zealand, and 18 Faculty of Civil and Environmental Engineering, Technion-Israel Institute of Technology, 19 Haifa, 32000, Israel.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Quttineh, Nils-Hassan
    Linköping University, Department of Mathematics, Optimization . Linköping University, Faculty of Science & Engineering.
    Adjustments of Public Transit Operations Planning Process for the Use of Electric Buses2017In: TRB Annual Meeting Online, 2017, Transportation Research Board , 2017Conference paper (Refereed)
    Abstract [en]

    This work investigates and discusses how the introduction of electric buses (EB), both battery and plug-in hybrid EB, will and should change the operations planning of a public transit system. It is shown that some changes are required in the design of a transit route network, and in the timetabling and vehicle scheduling processes. Other changes are not required, but are advisable, using this opportunity upon the introduction of EB. The work covers the main characteristics of different types of EB with a short description, including the most popular charging technologies, and it presents the generally accepted transit operations planning process. Likewise, it describes and analytically formulates new challenges that arise when introducing EB. The outcome of the analyses shows that multiple new considerations must take place. It is also shown that the different charging techniques will influence the operations planning process in different ways and to a varying extent. With overnight, quick and continuous charging, the main challenges are in the network route design step, given the possibility of altering the existing network of routes, with efficient and optimal changes of the timetabling and vehicle scheduling components.

  • 29.
    Quttineh, Nils-Hassan
    et al.
    Linköping University, Department of Mathematics, Optimization . Linköping University, Faculty of Science & Engineering.
    Häll, Carl Henrik
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ceder, Avi
    Department of Civil and Environmental Engineering, University of Auckland, New Zealand.
    Combined Timetabling and Vehicle Scheduling for Electric Buses2017In: Proceedings of the 22nd International Conference of Hong Kong Society for Transportation Studies (HKSTS), December 9-11, 2017, Hong Kong, China, Hong Kong: HKSTS , 2017, , p. 8Conference paper (Other academic)
    Abstract [en]

    In this paper we present a novel mathematical model, integrating the timetabling and vehicle schedulingproblems for electric buses. The objective is to minimize the number of buses while satisfying constraintsconcerning routing and charging, including design choices of where to install charging equipment. Weillustrate the different effects of tackling the timetabling and vehicle scheduling of electric buses as separateproblems or as a joint problem, both for fixed and variable headways. To do so, tests are performed with: (i) given timetable, i.e. solving only the vehicle scheduling problem, (ii) fixed headways for each line, (iii) variable headways. For these tests, a small case based on four bidirectional bus lines is used.

  • 30.
    Tsanakas, Nikolaos
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Olstam, Johan
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering. VTI.
    Emission estimation based on cross-sectional traffic data2017In: Prceedings of the 22nd International Transportation and Air Pollution Conference, 2017Conference paper (Refereed)
  • 31.
    Tsanakas, Nikolaos
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Olstam, Johan
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering. Statens väg- och transportforskningsinstitut.
    Hur fel det kan bli när man räknar emissioner baserat på statisk trafikdata2016Conference paper (Other academic)
    Abstract [sv]

    Vägtrafiken är en stor källa till utsläpp av både lokalt, regionalt och globalt förorenande ämnen. Lokalt och regionalt förorenande ämnen så som partiklar, kolmonoxid, kolväten och kväveoxider har påverkan på både natur och människa, och hälsoskadliga effekter i stadsmiljö är välkända. Globalt förorenande ämnen från biltrafiken, främst koldioxid, är ett stort problem ur ett klimatperspektiv. Hälsovådliga utsläpp är främst intressanta ur ett luftkvalitetsperspektiv, det är då inte de faktiska utsläppen från fordon (emissioner) som är mest intressanta utan halten förorenande ämnen i luften. Halten av förorenande ämnen beror naturligtvis på emissioner, men också på utsläpp från andra källor och väder. Genom en förenklad värdering av denna typ av utsläpp, där man tar hänsyn till lokala variationer i spridning av emissioner i luften, värderas luftkvalitet i samhällsekonomiska kalkyler i Sverige direkt baserat på fordonsemissioner. Mer detaljerat beräknas spridning av emissioner när kommuner gör luftkvalitetsberäkningar i verktyget SIMAIR. Koldioxid värderas direkt utifrån sina emissioner i samhällsekonomiska kalkyler.

    Oavsett om syftet är att göra samhällsekonomiska kalkyler eller mer detaljerade luftkvalitetsberäkningar i SIMAIR används i Sverige vanligen utdata från statiska trafikmodeller, baserade på användarjämvikt, som indata till emissionsberäkningarna. En statisk nätutläggning tar inte hänsyn till variationer över tid, eller utbredning av köer i trafiknätverket. Även om mer detaljerade trafikanalyser ofta görs med dynamiska trafikmodeller i senare planeringsskeenden, är inte resultatet från dessa analyser direkt tillämpbara i de emissionsberäkningar som görs i Samkalk (det verktyg som vanligen används för samhällsekonomiska kalkyler i Sverige) eller i SIMAIR.

    I Sverige används främst emissionsmodellen HBEFA som är en databas med emissionsfaktorer. Emissionsfaktorerna beskriver utsläpp i gram per fordonskilometer för en given fordonstyp, vägklass och trafikförhållande, som antingen definieras av flödesgränser eller hastighetsgränser. Emissionsfaktorer för fyra olika trafiksituationer finns definierade; ”free flow”, ”heavy”, ”saturated” och ”stop-and-go”. För att erhålla emissioner per timme på ett vägavsnitt multipliceras emissionsfaktorn med trafikflöde (fordon per timme) och vägavsnittets längd.

    När utdata från statiska trafikmodeller används som indata till emissionsberäkningar uppstår främst två problem. Statiska trafikmodeller medelvärdesbildar efterfrågan, och därmed trafikförhållanden, över en längre tid. Dessutom modellerar statiska modeller fördröjning till följd av köer på ett enskilt vägavsnitt, och köer kan inte propagera i ett vägnät. Potentiellt kan dessa två faktorer leda till att emissioner under- eller överskattas.

    För att undersöka omfattning och karaktären av dessa fel har trafikmätningar från delar av E4 genom Stockholm använts. Flödes- och hastighetsmätningar har aggregerats till 15 minutersperioder och använts som indata till emissionsberäkningar av CO2 (och därmed också energianvändning), CO, HC och NOx.

    Statiska trafikmodeller använder reseefterfrågan över en längre period (exempelvis en maxtimma) som indata. I statiska modeller förväntas lika många resor som påbörjas också avslutas, vilket kräver att tidsperioden är tillräckligt lång (i vårt fall med morgontrafiken i Stockholm innebär det aggregering över en tretimmarsperiod). Detta i sig gör att flöden i trafiknätverket är medelvärden för den studerade perioden, och topparna smetas därför ut. Det är dock flödestopparna som i verkligheten kraftigast bidrar till utsläppen. Jämförelsen mellan emissioner från aggregerade flöden och 15 minuters flöden ger en inblick i hur stort fel medelvärdesbildningen medför. Underskattningen av emissioner för samtliga utsläpp, utom HC, ligger mellan 10% och 15%. Utsläppen av HC underskattas med ca 20%.

    Vidare så analyserades effekten av att statiska modeller inte kan modellera uppbyggnad och avveckling av köer. HBEFA tillämpades få enligt den specifikation som Trafikverket tagit fram till Samkalk och SIMAIR. Sedan studerades, länk för länk, hur emissionsberäkningarna avviker mot när emissionsfaktorerna appliceras direkt på trafikmätningarna. På enskilda vägavsnitt kan underskattningen av utsläpp av HC och CO nå 40%, och totalt över hela den studerade vägsträckan underskattas utsläppen med 15% till 25% beroende på vilket ämne som avses.

    Slutligen har en metod för efterbearbetning av trafikdata tagits fram, baserad på den quasi-dynamiska nätutläggningsmetoden STAQ. Denna metod kan ses som en diskritiserad version av länktransmissionsmodellen, där förändringar i trafikförhållanden beskrivs som diskreta händelser i en simulering. STAQ använder fundamentaldiagram för att beskriva relationen mellan densitet, flöde och hastighet på varje vägsträcka, och kan delvis beskriva köers uppbyggnad och avveckling. En händelse i simuleringen motsvaras av att densiteten förändras på vägavsnittet, och därmed hastigheten. Däremot hanteras, genom de diskreta händelserna, tid på ett förenklat sätt, där trafikförhållanden på vägavsnitt endast registreras i samband med de diskreta händelserna. STAQ är framför allt utvecklad för att bättre bestämma fördröjning till följd av köer. I detta arbete har vi istället utvecklat ett angreppssätt som utnyttjar information som finns tillgänglig vid de diskreta händelserna för att förbättra beräkningen av emissioner. Med denna efterbearbetningsmetod underskattas emissionerna istället med cirka 10%, förutom för HC som underskattas med 20%.

    Vi har identifierat brister som uppstår när utdata från statiska trafikmodeller används som indata till emissionsberäkningar, och föreslagit en efterbearbetningsmetod för att försöka minska de fel som uppstår. Fortsatt arbete behöver fokusera på hur denna metodik fungerar i större nätverk och hur man bättre kan ta hänsyn till hur efterfrågan varierar inom en maxperiod.

  • 32.
    Tsanakas, Nikolaos
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Ekström, Joakim
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Olstam, Johan
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering. The Swedish National Road and Transport Research Institute, Linkoöping SE-581 95, Sweden.
    Reduction of errors when estimating emissions based on static traffic model outputs2017In: Transportation Research Procedia, ISSN 2324-9935, E-ISSN 2352-1465, Vol. 22, p. 10p. 440-449Article in journal (Refereed)
    Abstract [en]

    The rapid growth of traffic congestion has led to an increased level of emissions and energy consumption in urban areas. Well designed infrastructure and traffic controllers along with more efficient vehicles and policy measures are required to mitigate congestion and thus reduce transport emissions. In order to evaluate how changes in the traffic system affect energy use and emissions, traffic analysis tools are used together with emission models. In large urban areas emission models mainly rely on aggregated outputs from traffic models, such as the average link speed and flow. Static traffic models are commonly used to generate inputs for emission models, since they can efficiently be applied to larger areas with relatively low computational cost. However, in some cases their underlying assumptions can lead to inaccurate predictions of the traffic conditions and hence to unreliable emission estimates. The aim of this paper is to investigate and quantify the errors that static modeling introduces in emission estimation and subsequently considering the source of those errors, to suggest and evaluate possible solutions. The long analysis periods that are commonly used in static models, as well as the static models’ inability to describe dynamic traffic flow phenomena can lead up to 40 % underestimation of the estimated emissions. In order to better estimate the total emissions, we propose the development of a post processing technique based on a quasi-dynamic approach, attempting to capture more of the excess emissions created by the temporal and spatial variations of traffic conditions.

1 - 32 of 32
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • oxford
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf