Appraisal methodologies and the limits to speed gains

Increases in both speed and economic prosperity are an outstanding feature of human history. But are we close to a kind of decreasing returns of speed gains? The objective of the paper is to explain why speed increase and time gains are relevant to improve the social welfare. But sometimes, time gains are too costly, namely when time gains imply a very high cost for the community, at once monetary costs for public budgets and external cost for the environment. In order to enlighten the coming decreasing returns of speed, we test the concepts of "effective speed" and "socially effective speed". On the basis of "socially effective speed", transport investment appraisals are guided not purely by absolute speed, but by the type of service provided to users. For instance, what if transport services lead to public subsidies of several tens of euros per person per day? In sum, the construct of socially effective speed assists policy-makers in making appropriate investment choices concerning, namely in France, high speed rail projects that are, very often, not an appropriate response to travel demand. It is also possible to apply socially effective speed to urban mobility policies. Up to what extent is it relevant to increase the public transit supply in peri-urban areas?