  Heavy traffic congestion daily occurs at merging sections on highway. For
releasing this congestion, possibility of alternative configuration of vehicles
on multiple-lane road is discussed in this paper. This is the configuration
where no vehicles move aside on the other lane. It has a merit in making smooth
merging at an intersection or a junction due to so-called the "zipper effect".
We show, by developing a cellular automaton model for multiple lanes, that this
configuration is achieved by simple local interactions between vehicles
neighboring each other. The degree of the alternative configuration in terms of
the spatial increase of parallel driving length is studied by using both
numerical simulations and mean field theory. We successfully construct a
theoretical method for calculating this degree of the alternative configuration
by using cluster approximation. It is shown that the theoretical results
coincide with those of the simulations very well.
