  Application of thermodynamics to driven systems is discussed. As particular
examples, simple traffic flow models are considered. On a microscopic level,
traffic flow is described by Bando's optimal velocity model in terms of
accelerating and decelerating forces. It allows to introduce kinetic,
potential, as well as total energy, which is the internal energy of the car
system in view of thermodynamics. The latter is not conserved, although it has
certain value in any of two possible stationary states corresponding either to
fixed point or to limit cycle in the space of headways and velocities. On a
mesoscopic level of description, the size n of car cluster is considered as a
stochastic variable in master equation. Here n=0 corresponds to the fixed-point
solution of the microscopic model, whereas the limit cycle is represented by
coexistence of a car cluster with n>0 and free flow phase. The detailed balance
holds in a stationary state just like in equilibrium liquid-gas system. It
allows to define free energy of the car system and chemical potentials of the
coexisting phases, as well as a relaxation to a local or global free energy
minimum. In this sense the behaviour of traffic flow can be described by
equilibrium thermodynamics. We find, however, that the chemical potential of
the cluster phase of traffic flow depends on an outer parameter - the density
of cars in the free-flow phase. It allows to distinguish between the traffic
flow as a driven system and purely equilibrium systems.
