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| 2. |
- Frejinger, Emma, et al.
(författare)
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Sampling of alternatives for route choice modeling
- 2009
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Ingår i: Transportation Research Part B. - : Elsevier BV. - 0191-2615 .- 1879-2367. ; 43:10, s. 984-994
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a new paradigm for choice set generation in the context of route choice model estimation. We assume that the choice sets contain all paths connecting each origin-destination pair. Although this is behaviorally questionable, we make this assumption in order to avoid bias in the econometric model. These sets are in general impossible to generate explicitly. Therefore, we propose an importance sampling approach to generate subsets of paths suitable for model estimation. Using only a subset of alternatives requires the path utilities to be corrected according to the sampling protocol in order to obtain unbiased parameter estimates. We derive such a sampling correction for the proposed algorithm. Estimating models based on samples of alternatives is straightforward for some types of models, in particular the multinomial logit (MNL) model. In order to apply MNL for route choice, the utilities should also be corrected to account for the correlation using, for instance, a path size (PS) formulation. We argue that the PS attribute should be computed based on the full choice set. Again, this is not feasible in general, and we propose a new version of the PS attribute derived from the sampling protocol, called Expanded PS. Numerical results based on synthetic data show that models including a sampling correction are remarkably better than the ones that do not. Moreover, the Expanded PS shows good results and outperforms models with the original PS formulation.
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| 3. |
- Balakrishna, Ramachandran, et al.
(författare)
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A Simulation-based Evaluation of Advanced Traveler Information Systems
- 2005
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Ingår i: Transportation Research Record. - : Transportation Research Board of the National Academies. - 0361-1981 .- 2169-4052. ; 1910, s. 90-98
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Tidskriftsartikel (refereegranskat)abstract
- Traveler information has the potential to reduce travel times and improve their reliability. Studies have verified that driver overreaction from the dissemination of information can be eliminated through prediction-based route guidance that uses short-term forecasts of network state. Critical off-line tests of advanced dynamic traffic assignment-based prediction systems have been limited, since the system being evaluated has also been used as the test bed. This paper outlines a detailed simulation-based laboratory for the objective and independent evaluation of advanced traveler information systems, a laboratory with the flexibility to analyze the impacts of various design parameters and modeling errors on the quality of the generated guidance. MITSIMLab, a system for the evaluation of advanced traffic management systems, is integrated with Dynamic Network Assignment for the Management of Information to Travelers (DynaMIT), a simulation-based decision support system designed to generate prediction-based route guidance. Evaluation criteria and requirements for the closed-loop integration of MITSIMLab and DynaMIT are discussed. Detailed case studies demonstrating the evaluation methodology and sensitivity of DynaMIT's guidance are presented.
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| 4. |
- Balakrishna, R., et al.
(författare)
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Calibration of Microscopic Traffic Simulation Models : Methods and Application
- 2007
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Ingår i: Transportation Research Record. - : SAGE Publications. - 0361-1981 .- 2169-4052. ; :1999, s. 198-207
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Tidskriftsartikel (refereegranskat)abstract
- A mathematical framework and a solution approach are presented for the simultaneous calibration of the demand and supply parameters and inputs to microscopic traffic simulation models as well as a large-scale application emphasizing practical issues. Microscopic traffic simulation models provide detailed estimates of evolving network conditions by modeling time-varying demand patterns and individual drivers' detailed behavioral decisions. Such models are composed of elements that simulate different demand and supply processes and their complex interactions. Several model inputs (such as origin-destination flows) and parameters (car-following and lane-changing coefficients) must be specified before these simulation tools can be applied, and their values must be determined so that the simulation output accurately replicates the reality reflected in traffic measurements. A methodology is presented here for simultaneously estimating all microscopic simulation model parameters by using general traffic measurements. A large-scale case study for the calibration of the MITSimLab microscopic traffic simulation model by using the network of Lower Westchester County, New York, is employed to demonstrate the feasibility, application, and benefits of the proposed methodology.
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| 5. |
- Balakrishna, R., et al.
(författare)
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Time-dependent origin-destination estimation without assignment matrices
- 2019
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Ingår i: Transport Simulation: Beyond Traditional Approaches. - : Taylor & Francis Group. ; , s. 201-213
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Time-dependent origin-destination (OD) flows are crucial inputs to dynamic traffic assignment (DTA) models. However, they are often unobserved, and must be estimated from indirect traffic measurements collected from the study network. Approaches to estimate OD flows from link counts traditionally rely on assignment matrices that map the OD flow variables onto the counts. However, this method (a) approximates the complex relationship between OD flows and counts with a linear function, (b) is restricted to the use of only counts, and cannot exploit richer data such as speeds, densities or travel times, and (c) cannot estimate route choice and supply parameters that critically impact the OD estimates. This chapter presents a dynamic OD estimation method that is accurate and flexible in the use of general traffic data. Moreover, it simultaneously estimates all parameters with an impact on OD estimation, and can be applied to any traffic assignment model.
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| 6. |
- Ben-Akiva, Elana, et al.
(författare)
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Biodegradable lipophilic polymeric mRNA nanoparticles for ligand-free targeting of splenic dendritic cells for cancer vaccination
- 2023
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences (PNAS). - 0027-8424 .- 1091-6490. ; 120:26
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Tidskriftsartikel (refereegranskat)abstract
- Nanoparticle (NP) -based mRNA cancer vaccines hold great promise to realize person-alized cancer treatments. To advance this technology requires delivery formulations for efficient intracellular delivery to antigen-presenting cells. We developed a class of bioreducible lipophilic poly(beta- amino ester) nanocarriers with quadpolymer architec-ture. The platform is agnostic to the mRNA sequence, with one -step self-assembly allow-ing for delivery of multiple antigen-encoding mRNAs as well as codelivery of nucleic acid-based adjuvants. We examined structure-function relationships for NP-mediated mRNA delivery to dendritic cells (DCs) and identified that a lipid subunit of the pol-ymer structure was critical. Following intravenous administration, the engineered NP design facilitated targeted delivery to the spleen and preferential transfection of DCs without the need for surface functionalization with targeting ligands. Treatment with engineered NPs codelivering antigen-encoding mRNA and toll -like receptor agonist adjuvants led to robust antigen-specific CD8+ T cell responses, resulting in efficient antitumor therapy in in vivo models of murine melanoma and colon adenocarcinoma.
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| 7. |
- Ben-Akiva, M., et al.
(författare)
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Towards Disaggregate Dynamic Travel Forecasting Models
- 2007
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Ingår i: Tsinghua Science and Technology. - 1007-0214. ; 12:2, s. 115-130
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Tidskriftsartikel (refereegranskat)abstract
- The authors argue that travel forecasting models should be dynamic and disaggregate in their representation of demand, supply, and supply-demand interactions, and propose a framework for such models. The proposed framework consists of disaggregate activity-based representation of travel choices of individual motorists on the demand side integrated with disaggregate dynamic modeling of network performance, through vehicle-based traffic simulation models on the supply side. The demand model generates individual members of the population and assigns to them socioeconomic characteristics. The generated motorists maintain these characteristics when they are loaded on the network by the supply model. In an equilibrium setting, the framework lends itself to a fixed-point formulation to represent and resolve demand-supply interactions. The paper discusses some of the remaining development challenges and presents an example of an existing travel forecasting model system that incorporates many of the proposed elements.
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| 8. |
- Cameron, D., et al.
(författare)
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Replica Management in the European Data Grid Project
- 2004
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Ingår i: Journal of Grid Computing. - 1570-7873 .- 1572-9184. ; 2:4, s. 341-351
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Tidskriftsartikel (refereegranskat)abstract
- Within the European DataGrid project, Work Package 2 has designed and implemented a set of integrated replica management services for use by data intensive scientific applications. These services, based on the web services model, enable movement and replication of data at high speed from one geographical site to another, management of distributed replicated data, optimization of access to data, and the provision of a metadata management tool. In this paper we describe the architecture and implementation of these services and evaluate their performance under demanding Grid conditions.
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