  The scientific field of traffic engineering encompasses a rich set of
mathematical techniques, as well as researchers with entirely different
backgrounds. This paper provides an overview of what is currently the
state-of-the-art with respect to traffic flow theory. Starting with a brief
history, we introduce the microscopic and macroscopic characteristics of
vehicular traffic flows. Moving on, we review some performance indicators that
allow us to assess the quality of traffic operations. A final part of this
paper discusses some of the relations between traffic flow characteristics,
i.e., the fundamental diagrams, and sheds some light on the different points of
view adopted by the traffic engineering community.
