With the emerging demand for road safety and entertainment applications, efficient information exchanges among vehicles in Vehicular Ad-hoc Networks (VANETs) have attracted considerable attention. Since VANETs usually suffer from intermittent connectivity, long delay, and packet loss, Road-Side Unit (RSU) placement has been introduced to improve communication performance recently. Although some researchers have focused on scheduling and routing of buses, efficient communication in public transportation systems with infrastructure has not been well studied. In this paper, we use a space-time graph to model topology changes in Bus Ad-hoc Networks (BANETs), and then propose an effective greedy algorithm to minimize the number of RSUs, such that the communication delay between any two buses is within a given delay bound in BANETs. To evaluate the proposed scheme, we conduct simulations and analyze the performance. The simulation results show that our algorithm can significantly reduce the number of installed RSUs and the average end-to-end delay in the entire network.