L'influence de la biactivité sur les stratégies spatiales des ménages

A survey conducted among staff at the University of Paris xii Val-de-Marne has shown that there are considerable differences in residential location and commuting strategies between those in one income households and those in two income households. This difference varies depending on social group. In the case of higher social groups, being in a two income household as opposed to a one income household increases the range of residential possibilities rather than the household's income per consumer unit, and in particular allows them to live nearer their work. In contrast, for low income groups, working couples commute considerably greater distances than persons in one income households. Thus, for lower income groups, belonging to a working couple mainly provides greater opportunities for home ownership, as an increase in distances to work does not affect journey times.     

Analyse de la prise en compte de la sécurité routière dans les plans de déplacements urbains

At the time when Urban Travel Plans (utps) are being introduced it is important to see how they take safety into account, certu's analysis of a dozen recent docuements shows the position occupied by road safety in utps, reveals the stated aims of the utps, identifies local actions aimed at improving road safety and highlights current or foresedable problems in the implementation of utps. The analysis shows that the way safety is taken into account varies considerably from one city to another. The desire to introduce measures to improve safety is usually present, but there is uncertainty as to how to tackle the issue. The examples of methods and actions that are given in this paper may provide some ideas which merit further examination for utps in the future. It is already obvious that utps will be responsible for profound changes in urban travel and its safety, and the results obtained over the next five years will doubtless have important consequences for the cities of tomorrow.     

Fusion de données pour l'estimation des temps de parcours via la théorie de l'évidence

The emergence of new information technologies and the transformation that has occurred in traffic management have both increased drivers' already considerable need for road traffic information. The travel time is one of the forms in which this information is presented, and a number of systems are based on its dissemination. In this context, this indicator is used as a measure of the impedance (or cost) of routes on the network and/or a congestion indicator. This raises the problem of estimating travel times with an acceptable degree of accuracy, which is a particularly difficult task in urban areas as a result of difficultes of a theoretical, technical and methodological nature. Thus, in order to find out the traffic conditions that prevail on an urban road, the traffic sensors that are usually used to measure traffic conditions are ineffective under certain circumstances. New measurement devices (cameras, GPS or cellphone tracking, etc.) mean that other sources of data are increasingly used in order to supplement the information provided by conventional measurement techniques and improve the accuracy of travel) time estimates. As a result, travel time estimation becomes a typical data fusion problem. This study deals with a multisource estimate of journey times and attempts to provide a comprehensive framework for the utilization of multiple data and demonstrate the feasibility of a travel time estimation system based on the fusion of data of several different types. In this case two types of data are involved, data from conventional induction loop sensors (essentially flow and occupancy measurements) and data from probe vehicles. The selected modelling framework is the Dempster-Shafer Evidence Theory, which has the advantage of being able to take account of both the imprecision and uncertainty of the data. The implementation of this methodology has demonstrated that, in each case, better results are achieved with fusion than with methods based on a single source of data and that the quality of the information, as measured by correctly classified rates, improves as the degree of precision required of the estimate is increased.     

La surestimation de la distance intervéhiculaire dans le brouillard

The difficulties of driving in fog are usually perceived in terms of lower sight distances and reduced ability to anticipate. However, the fall in the quality of visual information can also lead to errors in judgement by affecting the way drivers perceive their environment and their movement within it. We have conducted a series of experiments in a fog chamber and on a driving simulator, with the main aim of investigating the influence of fog on vehicle headway perception. Our results have revealed that distances are seriously overestimated in fog, and that this is likely to contribute to lower headways under such conditions, In fog an overestimation of headway between 20 and 55% was observed when vehicles were visible only as a result of their rear lights, which is the case in night-time or thick daytime fog. Under these conditions, the characteristics of vehicle lights become extremely important. Overestimation of distance becomes greater when vehicles have only one rear light instead of two, and when their two reac lights are close together. Perception of vehicle distance is therefore improved by two rear lights, placed as far apart as possible to indicate the dimensions of the vehicle. No significant overestimation of headways was found during thin or moderate daytime fog, when the vehicle outline remained visible. The results have shown the importance of the familiar size of vehicles as an indicator of distance in the situations we have investigated. They confirm the hypothesis that basic mechanisms of space perception are modified in fog.     

Utilisation de l'intelligence artificielle distribuée pour la simulation microscopique d'un carrefour

This paper describes an original reasoning mechanism based on the principles of Distributed Artificial Intelligence (PAI). The purpose of this research was to construct a microscopic simulator of the road traffic near an intersection. In this model vehicles are considered as agents whose knowledge base is constructed dynamically before each decision is taken. Knowledge is represented using the production rules formalism. Each element in the environment has conduct rules which are transmitted to all the vehicles which enter its sphere of action. The knowledge base of a vehicle is therefore the result of a concatenation and hierarchization of conduct rules for solely those elements which can influence its bejaviour. A masking procedure is used to implement the priorities which are defined by the scheduling of knowledge and allow the vehicle to have reflex responses. Although we have used just a few easily implemented conduct rules the first results from the simulator have achieved a good standard of realism. This research programme has created an interface which can be used to implement rules and configure and control the simulator.     

Évaluation des logiciels critiques: application à la méthode B

From the point of view of safety the bottleneck in the operation of computerized systems currently appears to be the software. In order to improve the safety of software an independent assessment of its quality has been advocated for several years. The assessment of safety critical software consists of a technical examination on the basis of a set of criteria by a body which is independent from the designer. These assessment criteria are derived from the standards which are in force, the state of the art and experience. The objective of an assessments to verify that the product meets the specified safety and functionality requirements. In this paper the authors describe a combined approach to the assessment of safety critical software. On the one hand, the process used in order to develop the software is examined, and on the other the quality of the end product (software) is checked. The first part of the paper will present the position of safety critical software in the context of railway standardization as well as the main assessment techniques with particular reference to that used at Inrets. The second part of the paper will deal with the contribution of formal methods such as the B method to the development of safety critical software. The software assessment aspect will pay particular attention to assessment of the product — analysis of the traceability of requirements, integration and validation of safety properties. The paper ends with a brief account of the extraction and interpretation of metrics for a formal development. Finally the criteria used to assess the process are rapidly described. This approach has been used in the framework of an Esprit project known as Cascade (Certification and Assessment of Safety Critical Application Development).     

Utilisation de modeles linéaires généralisés pour tester l'effet sur la sécurité d'une modification d'infrastructure comparaison des glissières en métal aux barrières en béton en terre-plein central d'autoroute

Generalized linear models provide a unifying framework which permits application of most of the interential methods that can be used to evaluate the impact of a road safety, measure. When observations are available before and after the introduction of the measure (i.e., when there is an experimental sample and a control sample which covers the same period which indicates what would have hapened if the measure had not been introduced, the difference between the two samples can be characterized d'using the appropriate interaction term. The models used estimate relative risk or incidence rates and are able to take into account a reasonable number of confounding factors. These methods have been applied in order to compace the severity of impacts with metal guardrails or concreté safety barriers on motorway central reservations. Poisson regression modelling has revealed a tendency for the risk of injury to be higher on road sections where concréte devices have a recently been installed. Multivariate analysis using logistic regression with adjustment with reference to certain factor for example a change in the number of lanes or the occurrence of a second impact shows that concrete barrier significantly increase (by a factor or about 1.9) the risk of injury in the case of a first impact on the central reservation. This estimate could be used in a more general cost benefit analysis.     

Prospective de la mobilité en Île-de-France Les apports et les limites d' une démarche économétrique

The author has used econometric equations to forecast travel by car and public transport in the Paris Region in the year 2020. The parameters of the equations were estimated on the basis of the observed trends since 1980. The technique is innovative in that it takes account of changes in structural factors (distribution of population by age, ring zone of residence and car ownership) using what are termed structural demand variables. Existing forecasts of these structural factors have been used. By applying hypotheses about changes in the remaining explanatory variables (income, transport price and supply) we have estimated traffic levels in a business as usual scenario whose sensitivity to the explanatory factors has also been tested. From this we have derived some additional indicators: network congestion, trip frequencies for each mode, travel time and aggregate quality indicators. Finally, some limits of this approach are discussed.     

Les objectifs de qualité urbaine et de sécurité routière dans la planification de la mobilit'e plan urbain de trafic (Brescia) et plan de déplacements urbains (Aix-en-Provence), deux visions différentes

Apart from the fact that it is of interest in itself to conduct the first comparison between Urban Travel Plans in France and Italy, this paper has the more general aim of examining a number of ideas which are shared by the scientific community, namely that urban policies and planning practices take ittle account of, or even completely ignore, road safety, the relationship between town planning and transport planning and consultation.