  One common feature of a vehicle, an ant and a kinesin motor is that they all
convert chemical energy, derived from fuel or food, into mechanical energy
required for their forward movement; such objects have been modelled in recent
years as {\it self-driven} ``particles''. Cytoskeletal filaments, e.g.,
microtubules, form a ``rail'' network for intra-cellular transport of vesicular
cargo by molecular motors like, for example, kinesins. Similarly, ants move
along trails while vehicles move along lanes. Therefore, the traffic of
vehicles and organisms as well as that of molecular motors can be modelled as
systems of interacting self-driven particles; these are of current interest in
non-equilibrium statistical mechanics. In this paper we point out the common
features of these model systems and emphasize the crucial differences in their
physical properties.
