Vehicular ad-hoc networks (VANETs) are a new kind of mobile ad hoc network, and applications over VANETs are emerging with bright prospects. Considering that a long-distance data dissemination mechanism is indispensable for most VANET applications, this paper presents Ara, a RoadSide Unit (RSU)-aided unicast routing mechanism for VANETs, and evaluates its performance under different traffic scenarios. The experimental results show that Ara can keep high data delivery ratios, low delay and few message overheads even in the case of RSU failures. Moreover, the paper theoretically analyzes the data delivery delay of Ara by establishing an analytical model. Based on a vehicular microscopic flow mobility model, the analytical model can deduce the delay under the different RSU conditions. The model has been verified on its validation by simulation experiments and therefore it can be applied to predict the performance of data delivery through Ara.