Representation of the Freight Transport System
F Combes, F Leurent, Université Paris Est, LVMT FR
A systemic representation of the freight transport system. It clarifies the logistic requirements and constraints of the shippers and the carriers. An application to a road-side truck transport survey is detailed.
The freight transport system consists of numerous agents of many different kinds, driven by specific objectives and sharing complicated relationships; as such, it is a complex system. As a consequence, determining whether there are market imperfections, and if such is the case, which regulation could be applied, is not an easy matter. Spatialised freight transport demand models are designed as decision support systems, i.e. to facilitate such decisions.
Initially inspired from passenger transport demand models, these models are based on more or less sophisticated adaptations of the classic four steps model of the passenger transport system, i.e. the sequence of trip generation, trip distribution, mode choice and path choice.
Perhaps unsurprisingly, the most original and potentially fruitful recent contributions have been targeted to issues missed by the four steps model. Let us quote for example the SCGE (Spatialised Computable General Equilibrium) models, which are focused on the retroactive linkage between the economy and the demand for transport, the EUNET 2.0 model in which supply chains are represented in an explicit way, the Aggregate-Disaggregate-Aggregate model of Ben Akiva and de Jong in which the shipment size is explicitly taken into account, and the NODUS model which addresses the logistic constraints of the carriers and their consequences on transport costs.
However, these advances lack a common analysis framework, say a systemic representation of the freight transport system which would be both understandable, realistic, and convenient for model building. This paper is purported to provide such a representation. The economic agents at stake are identified, and so are their options, decisions and relations. Similar agents are grouped in categories, which are identified and described in the frame of a microeconomic paradigm. In particular, the proposed representation is endowed with three features:
* The shipment characteristics are crucial: they are indeed most important, both from the perspective of the logistic imperatives of the shipper and of the technical constraints of the carriers.
* The representation is a layered one: this stands for the hierarchical nature of the decisions involved. For example, a shipper asks for a transport service with given characteristics such as speed, reliability, safety, and cost. The transport techniques themselves should thus be addressed in an abstract way, with respect to these characteristics.
* The logistic requirements of the shippers and the technical constraints of the carriers are clearly stated, thus overcoming a general vagueness about the role of logistics in freight transport demand.
The representation has a number of implications. First, it is consistent with the recent modelling advances. Second, it provides a basis for theoretical improvements which would be significant in the aim of decision support. Third, it may suggest metrologic advances, i.e. methodological improvements in data collection.
The third point is illustrated as follows. To illustrate the third point, we adapted a road-side survey protocol for trucks, commonly used by the French Transport Ministry to gather data on the nature of the freight transported by road, its origin, destination, and amount, in order to investigate the actual effects of some logistical requirements put by the shippers on the carriers. In particular, three results are derived: first, the volume constraint plays an important role in road freight transport; as a consequence the statement that productivity in road freight transport has been decreasing over the past few years might be erroneous. Second, the logistic requirements of the shippers may induce the carriers to use specific organizations (two drivers in each truck, relay). Third, most often, these logistic requirements do not imply specific orders about delivery times (more precisely, the drivers state no difference between an expected delivery time and a compulsory delivery time.)
Association for European Transport