This article examines an official participation process; the public debate organised by the National Committee of Public Debate (CNDP) between March and June 2006. The debate questioned the interest of a project that some inhabitants and authorities were awaiting since 1965 to be realized; the construction of a motorway extension in the outskirts of Paris, France. Our analysis comprehends the examination of this debate referring to an assessment scheme for participatory processes. The assessment scheme distinguishes three types of effects, each one on a different scale of time: 1) Procedural effects concern short term effects of the debate, e.g. access and enrichment of information, conditions of public participation. 2) Substantial effects concern the impact of the debate on the final public decision. They can only be assessed once the decision is taken. 3) Contextual effects consider long term impacts of the debate on society and community, on civil as well as on institutional levels. Our study shows how these three types of effects interfere in our case study.