  In general, there are two kinds of cooperative driving strategies, planning
based strategy and ad hoc negotiation based strategy, for connected and
automated vehicles (CAVs) merging problems. The planning based strategy aims to
find the global optimal passing order, but it is time-consuming when the number
of considered vehicles is large. In contrast, the ad hoc negotiation based
strategy runs fast, but it always finds a local optimal solution. In this
paper, we propose a grouping based cooperative driving strategy to make a good
tradeoff between time consumption and coordination performance. The key idea is
to fix the passing orders for some vehicles whose inter-vehicle headways are
small enough (e.g., smaller than the pre-selected grouping threshold). From the
viewpoint of optimization, this method reduces the size of the solution space.
A brief analysis shows that the sub-optimal passing order found by the grouping
based strategy has a high probability to be close to the global optimal passing
order, if the grouping threshold is appropriately chosen. A series of
simulation experiments are carried out to validate that the proposed strategy
can yield a satisfied coordination performance with less time consumption and
is promising to be used in practice.
