Data sharing is an integral part of vehicular ad hoc networks (VANETs), which provide drivers with safe and comfortable driving environments. However, when data are shared among multiple vehicles, they must be encrypted multiple times. Some solutions have used identity-based broadcast encryption to solve this problem. However, these schemes have two major limitations. First, the decryption cost is linearly related to the number of data receivers, where the identity of other receivers must be known. Second, only the data sender can forward the data. To address these important deficiencies, we propose an identity-based broadcast proxy re-encryption scheme to realize flexible and efficient data-sharing in VANETs. The data sender generates a fixed ciphertext that can be obtained by newly added vehicles through authorized vehicles. Data receivers can decrypt ciphertext directly without knowing the identities of other receivers, where the decryption overhead is constant. In addition, our scheme can achieve complete anonymous data sharing to protect vehicle privacy. A security proof shows that our scheme has sufficient security, and a performance analysis shows that our scheme performs well. Our proposed scheme is thus suitable for securing VANETs.