Conventional traffic information systems based on client-server approaches allow car drivers to navigate real-time road traffics, but the cost for maintaining the centralized traffic centers is high. On the other hand, decentralized traffic information systems that traffic conditions are exchanged between vehicles via intervehicle communication (IVC) significantly reduce the construction cost and efforts. Unfortunately, while the penetration of vehicles with such the system is insufficient, the systems suffer from problems to disseminate traffic conditions in disconnected vehicular ad-hoc networks. Therefore, we propose a novel road traffic information system which utilizes both IVC and infrastructure-based peer-to-peer communications. The system is organized as a two-tier hierarchical architecture where all vehicles broadcast traffic conditions over lower-tier vehicular ad-hoc networks. Moreover, parts of vehicles with infrastructure communication facilities further form a high-tier peer-to-peer overlay and exchange traffic information and lookups to redeem the dis-connectivity of the vehicular ad-hoc networks. Simulation results demonstrate that the proposed system performs well in terms of both delay for traffic information lookup and message exchanging overheads.