  The knowledge of real-life traffic pattern is crucial for good understanding
and analysis of transportation systems. This data is quite rare. In this paper
we propose an algorithm for extracting both the real physical topology and the
network of traffic flows from timetables of public mass transportation systems.
We apply this algorithm to timetables of three large transportation networks.
This enables us to make a systematic comparison between three different
approaches to construct a graph representation of a transportation network; the
resulting graphs are fundamentally different. We also find that the real-life
traffic pattern is very heterogenous, both in space and traffic flow
intensities.
