Vehicular Ad-hoc Networks (VANETs) has been regarded as an emerging and promising network for transportation because it can provide safety and useful traffic information for passengers. VANET employs IEEE 802.11p standard as transmission protocol that defines media access control (MAC) and physical layers, where provide one control channel and six service channels. IEEE 802.11p MAC protocol is a contention-based protocol that has low channel utilization under heavy traffic load. In this paper, we propose a Distributed Channel Assignment Scheme (DCAS) to increase the successful rate of channel reservation and avoid hidden terminal problem to increase the channel utilization. Besides, we also propose an emergency state transition mechanism when VANETs need to transmit emergency messages. Through state transition, vehicles in normal state using non-safety applications will enter emergency state to rapidly forward emergency message to reduce transmission delay. Simulation results show that the proposed scheme can improve 27% channel utilization in average comparing with random channel selection scheme.