Cooperative ITS systems are expected to highly improve the efficiency of road mobility. Wireless communications are used by these systems to disseminate centralized real-time traffic information to radio-equipped vehicles. Current proposals for traffic information dissemination either exploit dedicated cellular transmissions to interested vehicles, or cooperatively relay the information through vehicular ad-hoc networks. However, dedicated cellular transmissions may pose energy cost and traffic scalability issues to network operators. On the contrary, purely ad-hoc solutions may suffer from network disconnections and not always ensure adequate service reliability. To overcome these limitations, this paper introduces RoAHD, a hybrid approach in which a few messages injected through the cellular system are followed by a cooperative multi-hop dissemination in the vehicular network. RoAHD exploits multi-hop road connectivity information obtained at a low channel cost. Thanks to this knowledge, it is capable to operate smart injection decisions to ensure good levels of message delivery.