In this paper, we consider infrastructure-to-vehicle cooperative communications in which roadside access points use vehicles as relaying terminals. This can be particularly useful in suburban or remote areas where the frequent deployment of roadside access points is not either possible or cost-effective. For the doubly-selective vehicular channel under consideration, we employ a precoded cooperative transmission technique to extract the underlying rich multipath-Doppler-spatial diversity. Under the assumption of decode-and-forward relaying, we derive a pairwise error probability expression and demonstrate the achievable diversity gains. We further provide Monte Carlo simulations to confirm the analytical derivations and provide insight into the error rate performance of infrastructure-to-vehicle cooperative communications.