  This work shows that the formation of a finite number of coalitions in a
nonatomic network congestion game benefits everyone. At the equilibrium of the
composite game played by coalitions and individuals, the average cost to each
coalition and the individuals' common cost are all lower than in the
corresponding nonatomic game (without coalitions). The individuals' cost is
lower than the average cost to any coalition. Similarly, the average cost to a
coalition is lower than that to any larger coalition. Whenever some members of
a coalition become individuals, the individuals' payoff is increased. In the
case of a unique coalition, both the average cost to the coalition and the
individuals' cost are decreasing with respect to the size of the coalition. In
a sequence of composite games, if a finite number of coalitions are fixed,
while the size of the remaining coalitions goes to zero, the equilibria of
these games converge to the equilibrium of a composite game played by the same
fixed coalitions and the remaining individuals.
