Vehicular ad hoc networks (VANETs) is an efficient enabler for safe and efficient Intelligent Transport Systems. The introduction of intermediate vehicles improves the performance of VANETs prominently. In this paper, we introduce buses as intermediate vehicles to improve the system efficiency. We also introduce the coalition game model to study the relationships between buses and add incentives for buses to cooperate with each other. According to theory of Shapley value, we find the solution to the buses coalition game satisfying efficiency symmetry and marginality. In the solution, buses that work as a coalition could obtain more payoff than they work alone. Thus, a bus is willing to cooperate with others and the coalition will maintain stability. The performance of VANETs will be improved due to the cooperation of buses. We have used the Opportunistic Network Environment (ONE) simulator to verify the high performance of the proposed mechanism in terms of higher delivery ratio.