Cuando los tecnicos y los politicos toman decisiones sobre proyectos y especialmente sobre proyectos de transporte existen sesgos. Aparte de las consecuencias en los plazos, costes y resultados de las obras, que ya de por si son bastante importantes, estos sesgos tienen efectos de profecias autocumplidas. Si un sesgo hacia un modo de transporte se mantiene durante un periodo de tiempo sostenido se puede desviar el trafico hacia ese modo de forma significativa y aunque no se consiga el resultado deseado o previsto, los costes hundidos son tan grandes que favoreces mucho a ese modo haciendo mucho mas difícil el desarrollo del modo de transporte realmente mas efectivo.
El ejemplo siguiente presenta el caso del sesgo sistematico en francia que favorece el transporte por carretera frente al transporte por ferrocarril. Tomado de un articulo de Damart y Roy (2009)
In that CBA (Cost Benefit Analysis) plays a crucial role in guiding a country’s strategic transport infrastructure decisions, the potential bias in the method can pose a problem. The following example from the domain of rail transport, specifically piggyback transport, is used to illustrate this point (Hammiche and Denant Boemont, 1997).
The SNCF, the French national railway company has long wanted to reinforce its competitive position in the goods transportation market. The present context seemed to favor a move in this direction. Indeed, road traffic has been increasing steadily for many decades. By 2010, the major roads and motorways will be completely congested, with trucks transporting goods representing a significant part of the total traffic flow. Piggyback transport—consisting of carrying trucks loaded with goods (either the tractor and the trailer or only the trailer) in railroad cars over long distances—is midway between road and rail transport. When the tractor and the trailer are carried together, load splitting is avoided by permitting the transport of goods to the points that can only be reached by truck. In addition, the high speed of the train (120 km/h according to Hammiche and Denant-Boemont) compensates for the transport of a dead load (the tractor). Loading/unloading times are reduced through innovative technical solutions. Moreover, though the transport is accomplished by train, this type of railway is nevertheless closer to the concept of motorway since the rail lines are linked to motorways at hubs, which can be compared to road junctions.
However, after examining the CBA studies carried out to compare the feasibility of road or rail infrastructure projects, Hammiche and Denant-Boemont concluded that the biases inherent to this procedure have led systematically to overestimating the return of road infrastructure projects and, conversely, to underestimating the return of rail transport projects using piggyback. The types of bias are many. They concern the external effects of road transport and the hypotheses about the pricing of road and rail transport, as well as its implications in terms of modal transfer.