| 1. |
- Vadeby, Anna, 1969-
(författare)
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Computer based statistical treatment in models with incidental parameters : inspired by car crash data
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Bootstrap and Markov chain Monte Carlo methods have received much attention in recent years. We study computer intensive methods that can be used in complex situations where it is not possible to express the likelihood estimates or the posterior analytically. The work is inspired by a set of car crash data from real traffic.We formulate and develop a model for car crash data that aims to estimate and compare the relative collision safety among different car models. This model works sufficiently well, although complications arise due to a growing vector of incidental parameters. The bootstrap is shown to be a useful tool for studying uncertainties of the estimates of the structural parameters. This model is further extended to include driver characteristics. In a Poisson model with similar, but simpler structure, estimates of the structural parameter in the presence of incidental parameters are studied. The profile likelihood, bootstrap and the delta method are compared for deterministic and random incidental parameters. The same asymptotic properties, up to first order, are seen for deterministic as well as random incidental parameters.The search for suitable methods that work in complex model structures leads us to consider Markov chain Monte Carlo (MCMC) methods. In the area of MCMC, we consider particularly the question of how and when to claim convergence of the MCMC run in situations where it is only possible to analyse the output values of the run and also how to compare different MCMC modellings. In Metropolis-Hastings algorithm, different proposal functions lead to different realisations. We develop a new convergence diagnostic, based on the Kullback-Leibler distance, which is shown to be particularly useful when comparing different runs. Comparisons with established methods turn out favourably for the KL.In both models, a Bayesian analysis is made where the posterior distribution is obtained by MCMC methods. The credible intervals are compared to the corresponding confidence intervals from the bootstrap analysis and are shown to give the same qualitative conclusions.
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| 2. |
- Romano, Luigi, 1994, et al.
(författare)
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Analytical results in transient brush tyre models: theory for large camber angles and classic solutions with limited friction
- 2022
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Ingår i: Meccanica. - : Springer Science and Business Media LLC. - 1572-9648 .- 0025-6455. ; 57:1, s. 165-191
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Tidskriftsartikel (refereegranskat)abstract
- This paper establishes new analytical results in the mathematical theory of brush tyre models. In the first part, the exact problem which considers large camber angles is analysed from the perspective of linear dynamical systems. Under the assumption of vanishing sliding, the most salient properties of the model are discussed with some insights on concepts as existence and uniqueness of the solution. A comparison against the classic steady-state theory suggests that the latter represents a very good approximation even in case of large camber angles. Furthermore, in respect to the classic theory, the more general situation of limited friction is explored. It is demonstrated that, in transient conditions, exact sliding solutions can be determined for all the one-dimensional problems. For the case of pure lateral slip, the investigation is conducted under the assumption of a strictly concave pressure distribution in the rolling direction.
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| 3. |
- Haraldsson, Mattias
(författare)
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Diskreta valmodeller för analys av urbana transport- och distributionssystem
- 2001
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Det arbete som presenteras här, syftar till att pröva ekonometriska analysmetoder för det tvärsnittsdata som samlas in i projektet. Metoderna appliceras på ett datamaterial som tidigare samlats in, och som i flera viktiga avseenden har en struktur som liknar det material som ska analyseras framledes. I notatet utförs analyser av detta material i syfte att testa logitmodellens lämplighet för det fortsatta forskningsarbetet. Utöver att testa logitmodellens värde bidrar arbetet också till en fördjupad analys av det befintliga materialet. Dessutom prövas en teknik för att ersätta saknade data.
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| 4. |
- Zhu, Li-wei, et al.
(författare)
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Fault Diagnosis On The Highway Surveillance And Control System Based On Petri Net Model
- 2013
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Ingår i: Proceedings of the 16th International Conference Road Safety on Four Continents. - Linköping : Statens väg- och transportforskningsinstitut.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Highway Surveillance and Control System (HSCS) is the critical system to improving the traffic safety and efficiency of highway. In this paper, the fault diagnosis model of HSCS based on Petri Net Model is provided in line with the composition, fault types and features of HSCS, and subsequently the fault diagnostic algorithm is given. In the end of this paper, the fault diagnosis of Microwave Vehicle Detector (MVD) in HSCS is presented to verifying the proposed fault diagnosis model and algorithm.
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| 5. |
- Brännlund, Ulf, et al.
(författare)
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Railway Timetabling Using Lagrangian Relaxation
- 1998
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Ingår i: Transportation Science. - : Institute for Operations Research and the Management Sciences (INFORMS). - 0041-1655 .- 1526-5447. ; 32:4, s. 358-369
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Tidskriftsartikel (refereegranskat)abstract
- We present a novel optimization approach for the timetabling problem of a railway company, i.e., scheduling of a set of trains to obtain a profit maximizing timetable, while not violating track capacity constraints. The scheduling decisions are based on estimates of the value of running different types of service at specified times. We model the problem as a very large integer programming problem. The model is flexible in that if allows for general cost functions. We have used a Lagrangian relaxation solution approach, in which the track capacity constraints are relaxed and assigned prices, so that the problem separates into one dynamic program for each physical train. The number of dual variables is very large. However, it turns out that only a small fraction of these are nonzero, wh ich one may take advantage of in the dual updating schemes. The approach has been, tested on a realistic example suggested by the Swedish National Railway Administration. This example contains 18 passenger trains and 8 freight trains to be scheduled during a day on a stretch of single track, consisting of 17 stations. The computation times are rather modest and the obtained timetables are within a few percent of optimality.
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| 6. |
- Vadeby, Anna
(författare)
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Modelling and inference of relative collision safety in cars
- 1998
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We propose a new mathematical model for relative collision safety in cars. Our present research is restricted to head-on crashes between two cars and we try to determine how much of the injury risk in a crash that depends on car model. The relative risks include the driver populations of the different car models. When two cars crash they are exposed to the same force, but the damage severity is different depending on various factors such as car mass, change of speed and design of the car. To explore the relative risks between different car models, we build a model where we let car mass, change of speed and design of the car explain the injury outcome in the crashes. The mathematical model we use is a birth process where we let the states correspond to the injury classes. A data base containing police reported traffic accidents and hospital information is used to explore the relationships in our model.A bootstrap analysis is made to produce a picture of the uncertainty of the estimates. The uncertainty from the bootstrap analysis is compared to the asymptotic estimate of the uncertainty given by the inverse of an information sub-matrix.
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| 7. |
- Forsman, Åsa
(författare)
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Prognosmodeller för antal dödade i vägtrafiken : en utvärdering av Poissonregression med seriellt korrelerade residualer
- 2008
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Rapport (populärvet., debatt m.m.)abstract
- Probability models play an important role when analysing and forecasting time series of the number of fatalities in road traffic accidents. This report presents a study of a method of analysing time series of count data that also can account for serial correlation. The method is a development of traditional Poisson regression where overdispersion and serial correlation is modelled by a latent process. The aim of the study was partly to test how the method worked in practice, partly to study the importance of considering possible serial correlation in the data. The method was tested both for the total number of killed road users and the number of killed road users divided in the following six categories: car occupants (drivers and passengers in passenger cars, lorries, and busses), motor-cyclists, mopedists, cyclists, pedestrians, and others.
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| 8. |
- Yu, Rongjie, et al.
(författare)
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Developing Hierarchical Bayesian Safety Performance Functions Using Real-Time Weather and Traffic Data
- 2013
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Ingår i: Proceedings of the 16th International Conference Road Safety on Four Continents. - Linköping : Statens väg- och transportforskningsinstitut.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Safety Performance Function (SPF) is essential in traffic safety analysis, and it is useful to unveil hazardous factors related to the crash occurrence. Recently, Bayesian inference technology has drawn many researchers’ interest as an alternative. Among those frequently used models, Hierarchical Bayesian (HB) models are the most popular ones. One reason that HB models are frequently used is that the data structures for crash frequency studies are originally hierarchical (e.g. segment level, seasonal level, and corridor level). Besides, HB models are powerful enough to solve the over-dispersion issue which usually exists in traffic safety data. In this study, four types of HB models are compared (Poisson-gamma model, Random effects Poisson-gamma model, Correlated Random effects Poisson-lognormal model, Uncorrelated Random effects Poisson-lognormal model) with the basic Poisson model. The study focuses on a 15-mile mountainous freeway on I-70 in Colorado. Crash occurrences are aggregated at the homogenous segmentation level and the whole segment was split into 120 homogenous segments. Moreover, real-time traffic data prior to each crash were archived by 30 Remote Traffic Microwave Sensors (RTMS) and real-time weather information are provided by 6 weather stations along the studied roadway. For the model evaluation methods, Deviance Information Criterion (DIC), which recognized as Bayesian generalization of AIC (Akaike information criterion, and standard errors of the estimated coefficients for the independent variables was selected as the evaluation measure to select the best model(s). Comparisons across the models indicate that the Correlated Random effect Poisson model is superior with the smallest DIC values and the least standard errors. Model results indicate that hazardous factors related to the crash occurrence on the roadway segment should be studied by season. For example, the average temperature variable has a distinct coefficient sign for the snow and dry seasons. Moreover, two different sets of parameters have been concluded. Finally, conclusions have shed some lights on designing Active Traffic Management (ATM) strategies: for the dry season, the management strategy should focus on speed control and harmonize, while for the snow season special attentions are needed for the adverse weather conditions.
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| 9. |
- Li, Xin, 1985, et al.
(författare)
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Prediction of Vertical Dynamic Vehicle–Track Interaction and Sleeper–Ballast Contact Pressure in a Railway Crossing
- 2020
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Ingår i: Lecture Notes in Mechanical Engineering. - Cham : Springer International Publishing. - 2195-4356 .- 2195-4364. - 9783030380762 ; , s. 397-403
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Konferensbidrag (refereegranskat)abstract
- The vertical dynamic vehicle–track interaction in a railway crossing is simulated in the time domain based on a moving Green’s function approach in combination with an implementation of Kalker’s variational method to solve the non-Hertzian, and potentially multiple, wheel–rail contact. The method is demonstrated by calculating the wheel–rail impact load and the sleeper–ballast contact pressure for a hollow-worn wheel profile passing over a nominal crossing geometry.
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