  Physical concepts developed to describe instabilities in traffic flows can be
generalized in a way that allows one to understand the well-known instability
of supply chains (the so-called ``bullwhip effect''). That is, small variations
in the consumption rate can cause large variations in the production rate of
companies generating the requested product. Interestingly, the resulting
oscillations have characteristic frequencies which are considerably lower than
the variations in the consumption rate. This suggests that instabilities of
supply chains may be the reason for the existence of business cycles. At the
same time, we establish some link to queuing theory and between micro- and
macroeconomics.
