In this paper, we investigate the performance of a single-relay assisted cooperative vehicular network in a highway traffic scenario. Source and relaying vehicles are assumed to be traveling in the same direction with similar speeds. This results in a relative velocity nearly equal to zero and leads to a frequency-flat and time-flat fading in source-to-relay link. On the other hand, source-to-destination and relay-to-destination links are modeled as doubly-selective fading. To handle spreading in time and frequency, we propose a precoded cooperative scheme to exploit delay and Doppler spreads to our advantage. Under the assumption of amplify-and-forward relaying with orthogonal cooperation protocol, we derive a pairwise error probability expression and demonstrate the achievable diversity gains. We further conduct Monte Carlo simulations to confirm the analytical derivations and present the error rate performance of the proposed scheme with imperfect channel estimation.