  The relation between the fundamental observables of traffic flow (i.e.,
vehicle density, flow rate, and average velocity) is of great importance for
the study of traffic phenomena. Probably the most common source of such data
are inductive loop detectors, which count the number of passing vehicles and
measure their speed. We will present an analysis of detector data collected by
more than 3000 loop detectors during the past three years on the motorway
network of the state of North Rhine-Westphalia. Besides presenting some general
aspects of traffic flow, our analysis focuses on the characteristics of
so-called high-flow states, i.e. traffic states where the flow rate exceeds 50
vehicles per minute and lane (3000 veh/h/lane). We investigate the duration,
frequency and other statistics of such states, the viability of the data and we
study the conditions under which they occur. The factors that influence the
existence of high-flow states in traffic are, for instance, the fraction of
slow vehicles (namely trucks), the motorway's general topology (e.g. number of
lanes), the hour of the day and day of the week. This information is directly
accessible from the detector data.
