Vehicular Ad hoc NETworks (VANETs), an important part of the Intelligent Transportation System (ITS), should provide the reliable safety message broadcasts and efficient non-safety message transmissions to vehicles. The IEEE 1609.4 MAC is designed for VANETs to support multi-channel operations, but it is neither reliable enough for the safety message broadcast nor efficient enough in the service channel (SCH) resources utilization. In this paper, we propose a Cooperative - Efficient - Reliable multi-channel MAC for VANETs, named CER-MAC, which supports both TDMA and CSMA schemes in accessing the control channel (CCH) and provides cooperation in broadcasting the safety messages. The CER-MAC allows vehicle nodes to send safety messages without collision on the CCH within their reserved time slots or the time slots permitted by the neighbors. Moreover, the CER-MAC utilizes the SCH resources during the control channel interval (CCHI) for the non-safety message transmissions. Compared to the current IEEE 1609.4, the proposed CER-MAC protocol outperforms in the reliability for the safety message broadcast and the efficiency for the service channel utilization.