  We discuss a phenomenological approach to the description of unstable vehicle
motion on multilane highways that could explain in a simple way such observed
self-organizing phenomena as the sequence of the phase transitions "free flow
-> synchronized motion -> jam" and the hysteresis in them.
  We introduce a new variable called order parameter that accounts for possible
correlations in the vehicle motion at different lanes. So, it is principally
due to "many-body" effects in the car interaction in contrast to such variables
as the mean car density and velocity being actually the zeroth and first
moments of the "one-particle" distribution function. Therefore, we regard the
order parameter as an additional independent state variable of traffic flow and
formulate the corresponding evolution equation governing the lane changing
rate.
  In this context we analyze the instability of homogeneous traffic flow
manifesting itself in both of the mentioned above phase transitions where and
endowing them with the hysteresis. Besides, the jam state is characterized by
the vehicle flows at different lanes being independent of one another.
