  Many complex networks are only a part of larger systems, where a number of
coexisting topologies interact and depend on each other. We introduce a layered
model to facilitate the description and analysis of such systems. As an example
of its application we study the load distribution in three real-life
transportation systems, where the lower layer is the physical infrastructure
and the upper layer represents the traffic flows. This layered view allows us
to capture the fundamental differences between the real load and commonly used
load estimators, which explains why these estimators fail to approximate the
real load.
