Roadside-unit caching in vehicular ad hoc networks for efficient popular content delivery Driven by both personal and commercial interests, fast popular content delivery, as one of the key services offered by vehicular ad-hoc networks (VANETs), has recently received considerable attention. Most existing work mainly focuses on the resource allocation such as transmit power or subcarrier assignment from the on-board units (OBUs) to the roadside units (RSUs). However, due to the limited backhaul capacity, great efforts still need to be taken for delivering large-size files such as videos and music to the high speed vehicles. Motivated by the recent work of pre-storing files in the cell-edge base stations, in this paper, we address the efficient content delivery problems in VANET by caching popular files in the RSUs with large storage capacity. The main objective is to minimize the average time that an OBU downloads a file. We propose three algorithms of allocating files to RSUs, in the optimal, sub-optimal, and greedy ways respectively, where the first one can achieve the best performance, and the greedy one has the lowest complexity. We also analyze the average downloading time performance in terms of the number of RSUs, storage capacity, and vehicle speed. Simulation results indicate that the proposed RSU caching methods can significantly reduce the file-downloading time, and thus, improve the content delivery efficiency.