This study provides a tool by means of which a road planner, usually a civil engineer, can achieve a durable infrastructure of high quality, acceptable to all parties concerned. The various elements which facilitate the work of planning are united into a modernized methodology for the elaboration of road projects, called a methodology of concert. After analysing the sort of problems encountered in the planning of road infrastructure (Chapter 1), the author classifies the main causes of these into a hierarchy. This brings to light many factors which may stem from the structure of the project, the environment or from the various participants. The modernization of the working methods of road planners is a highly practical idea based on case studies (Chapter 2). The particular study on which this work is based is the "Comparaison de variantes 1999", made on the A 144 road between the cities of Villeneuve and Le Bouveret. From an initial group of four variants, which were a source of much contention between the interested parties, an optimal and consensual solution, called "Solution COPIL", was evolved in seven months, by means of this study. The choice of this solution was based on a method of multicriteria decision analysis of a completely aggregational form. This used the weightings defined by all the political partners in the study. The present author was allowed to sit in on about fifteen working sessions of the Technical Group and of the Guiding Committee, which consists of the political partners. The examination of this particular case provided him with much valuable information about the process of elaboration of a road plan. The requirements and aims of road infrastructure are analysed (Chapter 3). Here, the focus is on the definition of individual and collective needs and the evolution of social expectation of the general public. The implementation of public policy is then examined. Particular emphasis is laid on the principal public policies whose spatial incidence affects the road plan : land distribution, transport and environment. Some attention is paid to the consideration of mobility, its value, characteristics and future developments. The difference between the standard and the norm of road infrastructure is described explicitly. The various stages of the process of elaboration of a road plan are analysed in depth. (Chapter 4) Particular attention is paid to the initial stages of impulsion for the elaboration of the project and of the identification of needs, since these influence strongly the quality and the acceptability of the project. The author then proposes that the various stages of road infrastructure be presented in the form of a life cycle of which the present study only concerns some aspects. Various special procedures are then analysed critically in order to extract propositions to be integrated into the methodology of concert. The type of partners in the road project and the characteristics of the relationships between them are discussed. (Chapter 5) The set of weightings made by the political partners in the case study "Comparaison de variantes 1999" are used to analyze the relations between the various participants in this study. A "representative profile of a participant" is defined from the examination of their individual weighting profiles. The notion of sustainability is presented (Chapter 6). After a recapitulation of definitions and of the reasons for this new idea, the principles of a durable mobility are presented. The practical aspects of this new paradigm of society for the planner are defined and discussed. They concern the inclusion of the life cycle in the analysis of the effects of a project, the weighing of interests from the three points of view of economy, social utility and the environment, public participation and the use of consultation right from the start of the study. Public participation (Chapter 7) is an important element in the elaboration of a road plan as it ensures its acceptation by the public. It may even allow the phenomenon of appropriation of a project by peripheral partners to develop. The various forms of public participation are presented in a table showing almost seventy which can affect a road project. The detailed description of some of these methods are given in appendix. Rules for applying the methodology of concert are then presented and the advantages and limits of this are discussed. After in-depth consideration of the decision procedure and its participants, with details of the various subjective and objective factors which may influence it, the study describes the methods of multicriteria decision analysis. (Chapter 8) Terminology is defined and the main types of these methods are presented. In this study, partial aggregation methods have been applied to the road infrastructure projects. These methods may be divided into three categories according to whether they are applicable to a situation of choice, of sorting or of order. A complete presentation is then given of the principles and ways of using the various methods of partial aggregation of the Electre family (I, II, III, IV, IS and Tri). For projects of road infrastructure, the method of partial aggregation Electre III is suggested. This method is based on the idea of fuzzy criteria, defined by thresholds of preference, indifference or of veto. It can be used with computer software which greatly simplifies the work of the planner. To illustrate the use of this method, Electre III have been applied to the case of "Comparaison de variantes 1999". The method was used on the set of weightings defined by the members of the Guiding Committee. The specific software is convivial and intuitively simple to use, so that the author was able to recommend it to the road planner. Information systems of spatial reference are described and their relevance for road projects in the establishment of optimal corridors or as a support for a method of multicriteria decision analysis. The methodology of concert for the elaboration of road plans is presented in detail (Chapter 9) in the form of successive flow diagrams illustrating the various stages of the procedure. These diagrams, based on an iterative process of study, provide the basis of the methodology of concert. They are intended for the road planner who follows them through and orients his planning according to his answers to the various questions contained there. In conclusion (Chapter 10), various prospects for the utilization of the proposed modernized methodology are indicated, together with the lines of possible future research suggested by the results and ideas contained in this study.