For smart traffic scenarios, communication between traffic participants is of high importance. Classical approaches (e.g. for information about congestions) employ a server-based architecture, which raises scalability and privacy concerns. In this paper, we propose OverDrive, a decentralized overlay-based geocast service that is applicable in smart traffic scenarios and not prone to the shortcomings of centralized designs. Information requests for points in geographic space are routed directly via traffic participants until they reach a node in the proximity of that point. In contrast to other approaches, our overlay is specifically tailored towards supporting mobile nodes - vehicles connected via cellular networks - and leverages their speed and direction for optimizing peering decisions and minimizing maintenance overhead. Exhaustive simulations in complex smart traffic scenarios show that OverDrive achieves high delivery ratios even in high mobility environments. At the same time, communication overhead is kept low, making OverDrive suitable for the use with cellular networks.