How difficult is it to go beyond Dupuit ?

This text develops a comparison between Dupuit era of ingénieurs économistes where the same person was both a decision maker and an economist, and the present era where there is a clear separation between the decision making world and the economic research world, the result being that economic research is less well linked to decision making. Applying this analysis to the case of transport, it shows how this separation leads to inefficiencies in the implementation of economic analysis for decisions. It appears that the level and nature of these inefficiencies, which varied along the history of the field, highly depends on the role of expertise and consultancies which ensure the links between the two worlds.