Equilibrium traffic assignment on large Virtual Networks: Implementation issues and limits for multi-modal freight transport

Multi-Modal freight models are traditionally built following the well known “four steps
model” in which generation, distribution, modal-split and assignment are seen as separated
modules. An alternative approach, now implemented in some software, is to represent the
multi-modal network by means of a “mono-modal” one, in which each particular transport
operation (loading or unloading operation, transhipments ...) is represented by a dedicated
“virtual link”, that represents a specific operation in the transportation chain.
This approach, promoted by several authors, often referenced to as “super networks” or
“virtual networks”, is proven to give interesting results, but has the drawback to generate
much larger networks than the pure geographic representation of the studied area. It has
also some kind of “hidden trap”, linked to transport distances, that will be presented in this
paper and that can only be solved using appropriate assignment techniques.
This paper presents some results obtained on a large multi-modal network, using different
equilibrium assignment algorithms, in order to test their ability to give an appropriate solution
to the “distance trap”. It however concludes that the implementation of classical equilibrium
assignment techniques leads to solutions that are barely different from the one obtained
by a simple all-or-nothing assignment, opening the way to alternative multi-flow solutions.