  The relation between flow and density is an essential quantitative
characteristic to describe the efficiency of traffic systems. We have performed
experiments with single-file motion of bicycles and compare the results with
previous studies for car and pedestrian motion in similar setups. In the
space-time diagrams we observe three different states of motion (free flow
state, jammed state and stop-and-go waves) in all these systems. Despite of
their obvious differences they are described by a universal fundamental diagram
after proper rescaling of space and time which takes into account the size and
free velocity of the three kinds of agents. This indicates that the
similarities between the systems go deeper than expected.
