In this paper, we consider the use of unlicensed Wi-Fi band for vehicular ad hoc networks. In particular, we focus on exploiting channelization to improve spatial reuse, and avoiding interference that is external to the vehicular network. To this end, we propose Cognitive Channel Hopping (CCH), a decentralized channel hopping protocol where nodes select their channels based on cognitively collected channel quality measurements, in a manner that the network's connectivity is maintained. Our evaluation shows that CCH can take advantage of multiple channels that are available to significantly improve the network performance over a single-channel network, and can effectively tolerate external interference.