Évaluation des impacts environnementaux d'un PDU et de leurs conséquences socio-économiques : développements méthodologiques et tests sur le PDU de Nantes Métropole – Eval-PDU

Mobility is at the heart of the stakes for urban sustainable development and transportation policies that are set up in cities increasingly incorporate environmental components. Urban mobility plans (PDU) are, in France, an essential tool of urban mobility policies. Their environmental features have received increasing attention during the last decade, so that environmental impacts assessment of the PDU actions is now compulsory. The Eval-PDU project aims at developing PDU environmental impact assessment methodologies. It leans towards the case of Nantes Metropole and its present PDU (2000-2010), following a research-action program conducted by the IRSTV in 2007-2008 with the urban community of Nantes. This situation is a good illustration of the need of local public authorities for rigorously based tools to assess a series of impacts (air quality, noise, ...) effectively associated with various actions (or groups of actions) they lead. Beyond the monitoring of objective indicators, it is a matter of understanding and quantifying a cascade of physical and social causalities and, further, its consequences for the quality of life and its perception by the inhabitants. The need concerns as much ex post evaluations of what has already been done, as ex ante evaluations of what is being planned. The selected approach is based on multi-factor numerical simulations (air quality, noise, energy consumption, greenhouse gas emission) representing a set of alternative scenarios (before/after, with/without) figuring changes in transportation generated by series of PDU actions. These environmental impact simulations are based on traffic simulations by means of a multimodal mobility model (especially road traffic, a key parameter for these environmental stakes). The traffic data allow to evaluate the consumption of energy and the emissions of atmospheric pollutants and greenhouse gases in the shape of an emission inventory. Modelling the interactions with the urban morphology further allows to evaluate and map the concentrations and exposures to pollutants. An analogous construction is used for the exposure to noise. Following, the assessment of the socio-economic consequences of these alternative simulations will help to produce synthetic indicators of the social benefits of the PDU actions for the environment. Several approaches will be tested. We will try to analyze the influences of air quality and noise – characterized by the results of the numerical simulations – on two types of socio-economical indicators : (1) well-being and declared satisfaction indicators, collected by an ad-hoc survey; (2) property values of housings. The econometric methods should allow to isolate the environmental factors effects among those of other preponderantly influent factors. In parallel, it is planned to explore alternative approaches, aiming at a first order assessment of some environmental impacts skipping all or parts of the numerical model chaining while keeping the capability of analysing the processes from the socio-economic point of view. The first tested approach consists in evaluating the environmental consequences of the changes in behaviour linked to one or several key actions of the PDU. It is expected to drive a survey among specific inhabitants especially concerned with these actions, to identify the individual strategies to adapt to transportation offer variations, especially as concerns individual car journeys. We will try to deduce a direct evaluation of the differential environmental impact on energy consumption and air quality. Finally, an alternative assessment method using composite indicators will also be tested. It will be applied to construct a health impact indicator of PDU actions, associated to atmospheric pollution, further compared to the results of the numerical models. This construction, based on a cascade of numerical models of the physical processes, coupled with surveys and geo-statistic analyses for the socio-economic aspects, raises a series of research questions: - the relevance of scenarios built on PDU actions; -the capability of a simulation model cascade, starting from the traffic model, to evaluate the environmental impacts of these actions; - the ability of relating the physical environmental indicators, established from the simulation models, with socio-economic indicators; - the assessment of some environmental impacts by alternative approaches without the complex models of the physical processes. Only strong interdisciplinarity allows to touch these questions on.